object.c:910
static VALUE
rb_mod_const_defined(mod, name)
    VALUE mod, name;
{
    ID id = rb_to_id(name);

    if (!rb_is_const_id(id)) {
	rb_name_error(id, "wrong constant name %s", rb_id2name(id));
    }
    return rb_const_defined_at(mod, id);
}
