Type.nextBase

Given a @leaf_type and a @root_type which is contained in its anchestry, 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.