leaf_type and a
root_type which is contained in its
ancestry, return the type that
root_type is the immediate parent
of. In other words, this function determines the type that is
derived directly from
root_type which is also a base class of
leaf_type. Given a root type and a leaf type, this function can
be used to determine the types and order in which the leaf type is
descended from the root type.
root_typeand the type to be returned.
Immediate parent of the returned type.