SelectMany.cs

public static IEnumerable<TResult> SelectMany<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, IEnumerable<TResult>> selector) { 
    if (source == null) throw Error.ArgumentNull("source");
    if (selector == null) throw Error.ArgumentNull("selector");
    return SelectManyIterator<TSource, TResult>(source, selector);
} 

static IEnumerable<TResult> SelectManyIterator<TSource, TResult>(IEnumerable<TSource> source, Func<TSource, IEnumerable<TResult>> selector) { 
    foreach (TSource element in source) { 
        foreach (TResult subElement in selector(element)) {
            yield return subElement;