Set#compare_by_identity

static VALUE
set_i_compare_by_identity(VALUE set)
{
    if (RSET_COMPARE_BY_IDENTITY(set)) return set;

    if (set_iterating_p(set)) {
        rb_raise(rb_eRuntimeError, "compare_by_identity during iteration");
    }

    return set_reset_table_with_type(set, &identhash);
}