@classmethod<EOL><INDENT>def _get_hierarchy(cls):<DEDENT>
bases = []<EOL>for base in cls.mro():  <EOL><INDENT>if hasattr(base, '<STR_LIT>'):<EOL><INDENT>bases.append(base)<EOL><DEDENT><DEDENT>del bases[-<NUM_LIT:1>]  <EOL>bases.reverse()<EOL>return bases<EOL>
Internal helper to return the list of polymorphic base classes.

        This returns a list of class objects, e.g. [Animal, Feline, Cat].