private static IEnumerable <TSource> ExceptImpl <TSource>(this IEnumerable <TSource> first, IEnumerable <TSource> second, IEqualityComparer <TSource> comparer)
{
var hashSetFromFirstSequence = new HashSet <TSource>(second, comparer);
return(first.SelectMany(element =>
hashSetFromFirstSequence.Add(element) ?
SelectManyEnumerable.Repeat(element, 1) :
SelectManyEnumerable.Empty <TSource>()));
}
public static IEnumerable <TSource> Where <TSource>(this IEnumerable <TSource> source, Func <TSource, bool> predicate)
{
return(source.SelectMany(elem => predicate(elem) ?
SelectManyEnumerable.Repeat(elem, 1) :
SelectManyEnumerable.Empty <TSource>()));
}