public function check_external_forbidden_words($ct, $val, $words)
 {
     $a2 = array('a:x^2)', 'b:(sin(x)+1)^2');
     $s2 = array();
     foreach ($a2 as $s) {
         $s2[] = new stack_cas_casstring($s);
     }
     $cs2 = new stack_cas_session($s2, null, 0);
     $at1 = new stack_cas_text($ct, $cs2, 0);
     $this->assertEquals($val, $at1->check_external_forbidden_words($words));
 }