Added modifier
Fixed iterators for binary trees. The problem now is that Iterator returns null
Changed the Collection returned by getChildren to a new Iterable Children. This allows it to provide an iterator that never returns null and an enumeration that might. This is so that binary trees can benifit from left or right positioned subtrees
Added documentation for Children
Renamed Children to SimpleCollection
Replaced the OptionalEnumeration with an Iterator
Simplified the collection
Strengthened doc on SimpleCollection. Implementations may need further work