System.Func.Invoke(TSource)

public static IEnumerable<TSource> DistinctBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
    {
        var seenKeys = new HashSet<TKey>();
        foreach (TSource element in source)
        {
            if (seenKeys.Add(keySelector(element)))
            {
                yield return element;
            }
        }
    }

public static Func<TSource, CancellationToken, TResult> WithCancellation<TSource, TResult>([NotNull] Func<TSource, TResult> lambda)
		{
			Contract.Requires(lambda != null);
			return (value, ct) =>
			{
				if (ct.IsCancellationRequested) ct.ThrowIfCancellationRequested();
				return lambda(value);
			};
		}

public static Func<TSource, CancellationToken, Task<TResult>> WithCancellation<TSource, TResult>([NotNull] Func<TSource, Task<TResult>> lambda)
		{
			Contract.Requires(lambda != null);
			return (value, ct) =>
			{
				if (ct.IsCancellationRequested) return FromCancellation<TResult>(ct);
				return lambda(value);
			};
		}

public async ValueTask<bool> MoveNextAsync()
            {
                if (_once)
                {
                    return false;
                }

                _once = true;

                while (await _source.MoveNextAsync())
                {
                    if (!_predicate(_source.Current))
                    {
                        Current = false;
                        return true;
                    }
                }

                Current = true;
                return true;
            }

public async ValueTask<bool> MoveNextAsync()
            {
                if (_once)
                {
                    return false;
                }

                _once = true;

                while (await _source.MoveNextAsync())
                {
                    if (!await _predicate(_source.Current))
                    {
                        Current = false;
                        return true;
                    }
                }

                Current = true;
                return true;
            }

public async ValueTask<bool> MoveNextAsync()
            {
                if (_once)
                {
                    return false;
                }

                _once = true;

                while (await _source.MoveNextAsync())
                {
                    if (_predicate(_source.Current))
                    {
                        Current = true;
                        return true;
                    }
                }

                Current = false;
                return true;
            }

public async ValueTask<bool> MoveNextAsync()
            {
                if (_once)
                {
                    return false;
                }

                _once = true;

                while (await _source.MoveNextAsync())
                {
                    if (await _predicate(_source.Current))
                    {
                        Current = true;
                        return true;
                    }
                }

                Current = false;
                return true;
            }

public async ValueTask<bool> MoveNextAsync()
            {
                Current = default;
                for (; ;)
                {
                    if (_inner == null)
                    {
                        if (await _source.MoveNextAsync())
                        {
                            _inner = _mapper(_source.Current).GetAsyncEnumerator(_ct);
                        }
                        else
                        {
                            return false;
                        }
                    }

                    if (await _inner.MoveNextAsync())
                    {
                        Current = _inner.Current;
                        return true;
                    }

                    await _inner.DisposeAsync();
                    _inner = null;
                }
            }

public async ValueTask<bool> MoveNextAsync()
            {
                Current = default;
                for (; ; )
                {
                    if (_inner == null)
                    {
                        if (await _source.MoveNextAsync())
                        {
                            _inner = _mapper(_source.Current).GetEnumerator();
                        }
                        else
                        {
                            return false;
                        }
                    }

                    if (!_ct.IsCancellationRequested && _inner.MoveNext())
                    {
                        Current = _inner.Current;
                        return true;
                    }

                    _inner.Dispose();
                    _inner = null;
                }
            }

private void NextHandler(Task<bool> t)
            {
                if (t.IsCanceled)
                {
                    ExceptionHelper.AddException(ref _error, new OperationCanceledException());
                    _sourceDone = true;
                    if (TryDispose())
                    {
                        ResumeHelper.Resume(ref _resume);
                    }
                } else
                if (t.IsFaulted)
                {
                    ExceptionHelper.AddException(ref _error, ExceptionHelper.Extract(t.Exception));
                    _sourceDone = true;
                    if (TryDispose())
                    {
                        ResumeHelper.Resume(ref _resume);
                    }
                }
                else if (t.Result)
                {
                    var cts = CancellationTokenSource.CreateLinkedTokenSource(_sourceCTS.Token);
                    IAsyncEnumerator<TResult> src;
                    try
                    {
                        src = _mapper(_source.Current).GetAsyncEnumerator(cts.Token);
                    }
                    catch (Exception ex)
                    {
                        ExceptionHelper.AddException(ref _error, ex);
                        _sourceDone = true;
                        src = null;
                        if (TryDispose())
                        {
                            ResumeHelper.Resume(ref _resume);
                            return;
                        }
                    }

                    if (src != null)
                    {
                        Interlocked.Increment(ref _disposeWip);
                        var inner = new InnerHandler(src, this, cts);
                        _inners.Enqueue(inner);

                        if (_disposeRequested)
                        {
                            while (_inners.TryDequeue(out var inner2))
                            {
                                inner2.Dispose();
                            }
                        }

                        if (TryDispose())
                        {
                            inner.MoveNext();
                            if (Interlocked.Decrement(ref _sourceOutstanding) != 0)
                            {
                                MoveNextSource();
                            }
                            ResumeHelper.Resume(ref _resume);
                        }
                    }
                }
                else
                {
                    _sourceDone = true;
                    if (TryDispose())
                    {
                        ResumeHelper.Resume(ref _resume);
                    }
                }
            }

public async ValueTask<bool> MoveNextAsync()
            {
                for (; ; )
                {
                    if (await _source.MoveNextAsync())
                    {
                        if (_collection.Add(_keySelector(_source.Current)))
                        {
                            return true;
                        }
                    }
                    else
                    {
                        return false;
                    }
                }
            }

public async ValueTask<bool> MoveNextAsync()
            {
                for (; ;)
                {
                    if (await _source.MoveNextAsync())
                    {
                        var key = _keySelector(_source.Current);
                        if (!_once)
                        {
                            _once = true;
                            _prevKey = key;
                            return true;
                        }
                        if (!_keyComparer.Equals(_prevKey, key))
                        {
                            _prevKey = key;
                            return true;
                        }

                        _prevKey = key;
                    }
                    else
                    {
                        return false;
                    }
                }
            }

private void Handle(Task<bool> task)
            {
                if (task.IsCanceled)
                {
                    AddException(new OperationCanceledException());
                    _done = true;
                    if (TryDispose())
                    {
                        Signal();
                    }
                }
                else if (task.IsFaulted)
                {
                    AddException(task.Exception);
                    _done = true;
                    if (TryDispose())
                    {
                        Signal();
                    }
                }
                else
                {
                    if (task.Result)
                    {
                        var v = _source.Current;

                        if (TryDispose())
                        {
                            var cts = CancellationTokenSource.CreateLinkedTokenSource(_sourceCTS.Token);
                            IAsyncEnumerator<TResult> innerSource;
                            try
                            {
                                innerSource = _mapper(v)
                                    .GetAsyncEnumerator(cts.Token);
                            }
                            catch (Exception ex)
                            {
                                _source.DisposeAsync();

                                AddException(ex);
                                _done = true;
                                Signal();
                                return;
                            }

                            var handler = new InnerHandler(this, innerSource, _prefetch, cts);
                            Interlocked.Increment(ref _allDisposeWip);
                            if (Add(handler))
                            {
                                handler.MoveNext();

                                if (Interlocked.Decrement(ref _outstanding) != 0)
                                {
                                    MoveNext();
                                }
                            }
                            else
                            {
                                // This will decrement _allDisposeWip so
                                // that the DisposeAsync() can be released eventually
                                DisposeOne();
                            }
                        }
                    }
                    else
                    {
                        _done = true;
                        if (TryDispose())
                        {
                            Signal();
                        }
                    }
                }
            }

public async ValueTask DisposeAsync()
            {
                if (!_done)
                {
                    for (; ; )
                    {
                        try
                        {
                            if (_groups.IsEmpty)
                            {
                                break;
                            }
                            if (await _source.MoveNextAsync())
                            {
                                var t = _source.Current;
                                var k = _keySelector(t);

                                var found = _groups.TryGetValue(k, out var g);

                                if (found)
                                {
                                    await g.Next(_valueSelector(t));
                                }
                            }
                            else
                            {
                                foreach (var gr in _groups)
                                {
                                    await gr.Value.Complete();
                                }
                                break;
                            }
                        }
                        catch (Exception ex)
                        {
                            foreach (var gr in _groups)
                            {
                                await gr.Value.Error(ex);
                            }

                            throw;
                        }
                    }
                }
                if (Interlocked.Decrement(ref _active) == 0)
                {
                    await _source.DisposeAsync();
                }
            }

public async ValueTask<bool> MoveNextAsync()
            {
                for (; ;)
                {
                    try
                    {
                        if (await _source.MoveNextAsync())
                        {
                            var t = _source.Current;
                            var k = _keySelector(t);

                            var found = _groups.TryGetValue(k, out var g);

                            if (!found)
                            {
                                g = new Group(k, this);
                                Interlocked.Increment(ref _active);
                                _groups.TryAdd(k, g);
                                Current = g;
                            }

                            await g.Next(_valueSelector(t));

                            if (!found)
                            {
                                return true;
                            }
                        }
                        else
                        {
                            foreach (var gr in _groups)
                            {
                                await gr.Value.Complete();
                            }
                            _done = true;
                            return false;
                        }
                    }
                    catch (Exception ex)
                    {
                        foreach (var gr in _groups)
                        {
                            await gr.Value.Error(ex);
                        }
                        _done = true;
                        throw;
                    }
                }
            }

public async ValueTask<bool> MoveNextAsync()
            {
                if (await _source.MoveNextAsync())
                {
                    Current = _mapper(_source.Current);
                    return true;
                }
                return false;
            }

public async ValueTask<bool> MoveNextAsync()
            {
                if (await _source.MoveNextAsync())
                {
                    Current = await _mapper(_source.Current);
                    return true;
                }
                return false;
            }

private void NextHandler(Task<bool> t)
            {
                if (t.IsCanceled)
                {
                    ExceptionHelper.AddException(ref _error, new OperationCanceledException());
                    _done = true;
                    if (TryDispose())
                    {
                        Signal();
                    }
                }
                else if (t.IsFaulted)
                {
                    ExceptionHelper.AddException(ref _error, ExceptionHelper.Extract(t.Exception));
                    _done = true;
                    if (TryDispose())
                    {
                        Signal();
                    }
                }
                else if (t.Result)
                {
                    var cts = CancellationTokenSource.CreateLinkedTokenSource(_sourceCTS.Token);
                    IAsyncEnumerator<TResult> src;
                    try
                    {
                        src = _mapper(_source.Current).GetAsyncEnumerator(cts.Token);
                    }
                    catch (Exception ex)
                    {
                        ExceptionHelper.AddException(ref _error, ex);
                        _done = true;
                        Dispose(_source);
                        Signal();
                        return;
                    }

                    if (TryDispose())
                    {
                        Interlocked.Increment(ref _allDisposeWip);
                        var inner = new InnerHandler(src, this, cts);

                        for (; ; )
                        {
                            var curr = Volatile.Read(ref _current);
                            if (curr == DisposedInnerHandler)
                            {
                                inner.Dispose();
                                break;
                            }
                            if (Interlocked.CompareExchange(ref _current, inner, curr) == curr)
                            {
                                curr?.Dispose();
                                inner.MoveNext();
                                MoveNext();
                                return;
                            }
                        }
                    }
                }
                else
                {
                    _done = true;
                    if (TryDispose())
                    {
                        Signal();
                    }
                }
            }

public static TSource ForgivingCaseSensitiveFind<TSource>(this IEnumerable<TSource> source, Func<TSource, string> valueSelector, string testValue)
        {
            if (source == null)
            {
                throw new ArgumentNullException(nameof(source));
            }
            if (valueSelector == null)
            {
                throw new ArgumentNullException(nameof(valueSelector));
            }

            var caseInsensitiveResults = source.Where(s => string.Equals(valueSelector(s), testValue, StringComparison.OrdinalIgnoreCase));
            if (caseInsensitiveResults.Count() <= 1)
            {
                return caseInsensitiveResults.SingleOrDefault();
            }
            else
            {
                // multiple results returned. now filter using case sensitivity
                var caseSensitiveResults = source.Where(s => string.Equals(valueSelector(s), testValue, StringComparison.Ordinal));
                return caseSensitiveResults.SingleOrDefault();
            }
        }

public static Dictionary<TKey, TValue> ToSoftDictionary<TSource, TKey, TValue>(
            this IEnumerable<TSource> source, Func<TSource, TKey> key, Func<TSource, TValue> value)
        {
            var dict = new Dictionary<TKey, TValue>();
            foreach (var item in source)
                dict[key(item)] = value(item);
            return dict;
        }

public static TSource MaxElement<TSource, R>(this IEnumerable<TSource> container, Func<TSource, R> valuingFoo) where R : IComparable
    {
        var enumerator = container.GetEnumerator();
        if (!enumerator.MoveNext())
            throw new ArgumentException("Container is empty!");

        var maxElem = enumerator.Current;
        var maxVal = valuingFoo(maxElem);

        while (enumerator.MoveNext())
        {
            var currVal = valuingFoo(enumerator.Current);

            if (currVal.CompareTo(maxVal) > 0)
            {
                maxVal = currVal;
                maxElem = enumerator.Current;
            }
        }

        return maxElem;
    }

public static TSource MinElement<TSource, R>(this IEnumerable<TSource> container, Func<TSource, R> valuingFoo) where R : IComparable
    {
        var enumerator = container.GetEnumerator();
        if (!enumerator.MoveNext())
            throw new ArgumentException("Container is empty!");

        var maxElem = enumerator.Current;
        var maxVal = valuingFoo(maxElem);

        while (enumerator.MoveNext())
        {
            var currVal = valuingFoo(enumerator.Current);

            if (currVal.CompareTo(maxVal) < 0)
            {
                maxVal = currVal;
                maxElem = enumerator.Current;
            }
        }

        return maxElem;
    }

public static IEnumerable<TSource> DistinctBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
        {
            var hash = new HashSet<TKey>();
            return source.Where(p => hash.Add(keySelector(p)));
        }

public static async ValueTask<int> CountAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, bool> predicate)
        {
            int count = 0;

            await foreach (TSource element in source)
            {
                checked
                {
                    if (predicate(element))
                    {
                        count++;
                    }
                }
            }

            return count;
        }

public static async ValueTask<long> LongCountAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, bool> predicate)
        {
            long count = 0;

            await foreach (TSource element in source)
            {
                checked
                {
                    if (predicate(element))
                    {
                        count++;
                    }
                }
            }

            return count;
        }

public static IEnumerable<TSource> DistinctBy<TSource, TKey>
    (this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
        {
            HashSet<TKey> seenKeys = new HashSet<TKey>();
            foreach (TSource element in source)
            {
                if (seenKeys.Add(keySelector(element)))
                {
                    yield return element;
                }
            }
        }

public static Func<TSource, ValueTask<TReturn>> Func<TSource, TReturn>(Func<TSource, ValueTask<TReturn>> func)
        {
            KeyValuePair<TSource, TReturn>? cache = null;
            var @lock = new AsyncLock();

            return async s =>
            {
                var tmpCache = cache;
                if (tmpCache.HasValue && Equals(tmpCache.Value.Key, s))
                    return tmpCache.Value.Value;

                using (await @lock.LockAsync())
                {
                    if (!cache.HasValue || !Equals(cache.Value.Key, s))
                        cache = new KeyValuePair<TSource, TReturn>(s, await func(s));

                    return cache.Value.Value;
                }
            };
        }

public static IEnumerable<TSource> DistinctBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
        {
            HashSet<TKey> knownKeys = new HashSet<TKey>();

            foreach (TSource element in source)
            {
                if (knownKeys.Add(keySelector(element)))
                {
                    yield return element;
                }
            }
        }

public static IEnumerable<TSource> IDistinctBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
        {
            var seenKeys = new HashSet<TKey>();
            return source.Where(element => seenKeys.Add(keySelector(element)));
        }

public static IEnumerable<SelectTryResult<TSource, TResult>> SelectTry<TSource, TResult>(this IEnumerable<TSource> enumerable, Func<TSource, TResult> selector)
        {
            foreach (TSource element in enumerable)
            {
                SelectTryResult<TSource, TResult> returnedValue;
                try
                {
                    returnedValue = new SelectTryResult<TSource, TResult>(element, selector(element), null);
                }
                catch (Exception ex)
                {
                    returnedValue = new SelectTryResult<TSource, TResult>(element, default(TResult), ex);
                }
                yield return returnedValue;
            }
        }

public static Dictionary<TKey, TSource> ToDictionaryWithCapacity<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
        {
            int capacity;

            if (source is ICollection<TSource> collection)
            {
                capacity = collection.Count;
            }
            else if (source is IReadOnlyCollection<TSource> readonlyCollection)
            {
                capacity = readonlyCollection.Count;
            }
            else
            {
                capacity = source.Count();
            }

            var result = new Dictionary<TKey, TSource>(capacity);

            foreach (var current in source)
            {
                result.Add(keySelector(current), current);
            }

            return result;
        }

public static IEnumerable<TSource> DistinctBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
        {
            HashSet<TKey> seenKeys = new HashSet<TKey>();
            foreach (TSource element in source)
            {
                if (seenKeys.Add(keySelector(element)))
                {
                    yield return element;
                }
            }
        }

public static TSource? MaxBy<TSource, TKey>(this IEnumerable<TSource> source, in Func<TSource, TKey> selector, IComparer<TKey>? comparer = null)
        {
            if (source is null) throw new ArgumentNullException(nameof(source));
            if (selector is null) throw new ArgumentNullException(nameof(selector));

            if (comparer is null) comparer = Comparer<TKey>.Default;

            using var enumerator = source.GetEnumerator();
            if (!enumerator.MoveNext()) return default;

            TSource maxElement = enumerator.Current;
            TKey maxKey = selector.Invoke(maxElement);

            while (enumerator.MoveNext())
            {
                TSource element = enumerator.Current;
                TKey key = selector.Invoke(element);

                if (comparer.Compare(key, maxKey) > 0)
                {
                    maxElement = element;
                    maxKey = key;
                }
            }

            return maxElement;
        }

public static TSource? MinBy<TSource, TKey>(this IEnumerable<TSource> source, in Func<TSource, TKey> selector, IComparer<TKey>? comparer = null)
        {
            if (source is null) throw new ArgumentNullException(nameof(source));
            if (selector is null) throw new ArgumentNullException(nameof(selector));

            if (comparer is null) comparer = Comparer<TKey>.Default;

            using var enumerator = source.GetEnumerator();
            if (!enumerator.MoveNext()) return default;

            TSource maxElement = enumerator.Current;
            TKey maxKey = selector.Invoke(maxElement);

            while (enumerator.MoveNext())
            {
                TSource element = enumerator.Current;
                TKey key = selector.Invoke(element);

                if (comparer.Compare(key, maxKey) < 0)
                {
                    maxElement = element;
                    maxKey = key;
                }
            }

            return maxElement;
        }

public static TResult? SelectFromAll<TSource, TResult>(this IEnumerable<TSource> source, in Func<TSource, TResult> selector, IEqualityComparer<TResult>? comparer = null)
        {
            if (source is null) throw new ArgumentNullException(nameof(source));
            if (selector is null) throw new ArgumentNullException(nameof(selector));

            if (comparer is null) comparer = EqualityComparer<TResult>.Default;

            using var enumerator = source.GetEnumerator();
            if (!enumerator.MoveNext()) return default;

            TResult result = selector(enumerator.Current);

            while (enumerator.MoveNext())
            {
                TResult current = selector(enumerator.Current);
                if (!comparer.Equals(result, current)) return default;
            }

            return result;
        }

public static int IndexOf<TSource>(this IList<TSource> source, Func<TSource, bool> predicate)
            where TSource : IEquatable<TSource>
        {
            for (int i = 0; i < source.Count; i++)
            {
                if (predicate(source[i]))
                {
                    return i;
                }
            }

            throw new ArgumentException("List doesn't contain such an element");
        }

[Pure]
    public static int IndexOf<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
    {
        Contract.Requires(source != null);
        Contract.Requires(predicate != null);

        int index = 0;
        foreach (TSource item in source)
        {
            if (predicate(item))
            {
                return index;
            }
            index++;
        }
        return -1;
    }

public static void Synchronize<TSource, TTarget>(this ObservableCollectionPlus<TTarget> targetCollection, IList<TSource> sourceCollection, Func<TSource, TTarget, bool> matcher, Func<TSource, TTarget> mapper) where TSource : IEquatable<TSource>
    {
        Contract.Requires(targetCollection != null);
        Contract.Requires(mapper != null);
        Contract.Requires(sourceCollection != null);
        Contract.Requires(sourceCollection.AllUnique());

        using (targetCollection.BeginMultiUpdate())
        {
            // move wrappers around to the right places
            // or create a new one
            for (int i = 0; i < sourceCollection.Count; i++)
            {
                var sourceItem = sourceCollection[i];
                var targetIndex = targetCollection.IndexOf(targereplacedem => matcher(sourceItem, targereplacedem));
                if (targetIndex >= 0)
                {
                    if (targetIndex != i)
                    {
                        Debug.replacedert(targetIndex > i, "this would only happen if we have duplicates...which we should never have!");
                        targetCollection.Move(targetIndex, i);
                    }
                    else
                    {
                        // NOOP - already in the right spot! :-)
                    }
                }
                else
                {
                    var newItem = mapper(sourceItem);
                    Debug.replacedert(matcher(sourceItem, newItem));
                    targetCollection.Insert(i, newItem);
                }
            }

            // Remove anything left
            while (targetCollection.Count > sourceCollection.Count)
            {
                targetCollection.RemoveLast();
            }

            Debug.replacedert(sourceCollection.Count == targetCollection.Count);
#if DEBUG
            for (int i = 0; i < sourceCollection.Count; i++)
            {
                Debug.replacedert(matcher(sourceCollection[i], targetCollection[i]));
            }
#endif
        }
    }

[Pure]
    public static IEnumerable<TSource> DistinctBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
    {
        HashSet<TKey> seenKeys = new HashSet<TKey>();
        foreach (TSource element in source)
        {
            if (seenKeys.Add(keySelector(element)))
            {
                yield return element;
            }
        }
    }

public bool Equals(TSource x, TSource y)
        {
            return _comparer.Equals(_keySelector(x), _keySelector(y));
        }

public int GetHashCode(TSource obj)
        {
            return _comparer.GetHashCode(_keySelector(obj));
        }

public static void Sort<TSource, TResult>( this TSource[] array, Func<TSource, TResult> selector ) where TResult : IComparable
		{
			Array.Sort( array, ( x, y ) => selector( x ).CompareTo( selector( y ) ) );
		}

public static void SortDescending<TSource, TResult>( this TSource[] array, Func<TSource, TResult> selector ) where TResult : IComparable
		{
			Array.Sort( array, ( x, y ) => selector( y ).CompareTo( selector( x ) ) );
		}

public static void Sort<TSource, TResult>( this TSource[] array, Func<TSource, TResult> selector1, Func<TSource, TResult> selector2 ) where TResult : IComparable
		{
			Array.Sort( array, ( x, y ) =>
			{
				var result = selector1( x ).CompareTo( selector1( y ) );
				return result != 0 ? result : selector2( x ).CompareTo( selector2( y ) );
			} );
		}

public static TSource MaxBy<TSource, TResult>
		(
			this IEnumerable<TSource> source,
			Func<TSource, TResult> selector
		)
		{
			var value = source.Max( selector );
			return source.FirstOrDefault( c => selector( c ).Equals( value ) );
		}

public static IEnumerable<TSource> MaxElementsBy<TSource, TResult>
		(
			this IEnumerable<TSource> source,
			Func<TSource, TResult> selector
		)
		{
			var value = source.Max( selector );
			return source.Where( c => selector( c ).Equals( value ) );
		}

public static TSource MinBy<TSource, TResult>
		(
			this IEnumerable<TSource> source,
			Func<TSource, TResult> selector
		)
		{
			var value = source.Min( selector );
			return source.FirstOrDefault( c => selector( c ).Equals( value ) );
		}

public static IEnumerable<TSource> MinElementsBy<TSource, TResult>
		(
			this IEnumerable<TSource> source,
			Func<TSource, TResult> selector
		)
		{
			var value = source.Min( selector );
			return source.Where( c => selector( c ).Equals( value ) );
		}

public static IEnumerable<TSource> WhereNot<TSource, TResult>
		(
			this IEnumerable<TSource> source,
			Func<TSource, bool> predicate
		)
		{
			return source.Where( c => !predicate( c ) );
		}

public static int Nearest<TSource>
		(
			this IEnumerable<TSource> self,
			int target,
			Func<TSource, int> selector
		)
		{
			var min = self.Min( c => Math.Abs( selector( c ) - target ) );
			return selector( self.FirstOrDefault( c => Math.Abs( selector( c ) - target ) == min ) );
		}