using System.Linq;

public static class Enumerable {

	public static System.Collections.Generic.IEnumerable<TOuter> Except<TOuter, TInner, TKey, TResult>(
		this System.Collections.Generic.IEnumerable<TOuter> outer,
		System.Collections.Generic.IEnumerable<TInner> inner,
		System.Func<TOuter, TKey> outerKeySelecter,
		System.Func<TInner, TKey> innerKeySelecter
	) {
		var comparer = System.Collections.Generic.EqualityComparer<TKey>.Default;

		var i = inner.ToDictionary( innerKeySelecter, comparer );
		return outer.Where(
			x => !i.ContainsKey( outerKeySelecter( x ) )
		);
	}

	public static System.Collections.Generic.IEnumerable<TOuter> Except<TOuter, TInner, TKey, TResult>(
		this System.Collections.Generic.IEnumerable<TOuter> outer,
		System.Collections.Generic.IEnumerable<TInner> inner,
		System.Func<TOuter, TKey> outerKeySelecter,
		System.Func<TInner, TKey> innerKeySelecter,
		System.Collections.Generic.IEqualityComparer<TKey> comparer
	) {
		var i = inner.ToDictionary( innerKeySelecter, comparer );
		return outer.Where(
			x => !i.ContainsKey( outerKeySelecter( x ) )
		);
	}

	public static System.Collections.Generic.IDictionary<K, V> ToDictionary<K, V>(
		this System.Collections.Generic.IEnumerable<V> collection,
		System.Func<V, K> keySelector,
		System.Collections.Generic.IEqualityComparer<K> keyComparer
	) {
		System.Collections.Generic.IDictionary<K, V> output = new System.Collections.Generic.Dictionary<K, V>( keyComparer );
		foreach ( var item in collection ) {
			output.Add( keySelector( item ), item );
		}
		return output;
	}

}