'Filters children by CAPA problem type, if configured'

def _filter_child(self, usage_key, capa_type):

    if (usage_key.block_type != 'problem'):
        return False
    descriptor = self.store.get_item(usage_key, depth=0)
    assert isinstance(descriptor, CapaDescriptor)
    return (capa_type in descriptor.problem_types)
