Calling either of these methods will nearly always
result in a ClassCastException. The reason is that
they expect extendedNamespace() to return a
collection of ModelElement instances, but actually
it returns a collection of StorableFeatureds.

Here is a patch. Note that in the process I
refactored nameIsValid() and
lookupElementExtended() to use a common helper
method for the search algorithm they share. This
way, nameIsValid() can take advantage of the index
that was previously only being used by
lookupElementExtended.