public function test_tricky_invalid_expression()
 {
     $this->setExpectedException('moodle_exception');
     $vs = new qtype_calculated_variable_substituter(array('a' => 1, 'b' => 2), '.');
     $vs->calculate('{a}{b}');
     // Have to make sure this does not just evaluate to 12.
 }
 /**
  * @expectedException moodle_exception
  */
 public function test_division_by_zero_expression()
 {
     if (intval(PHP_VERSION) < 7) {
         $this->markTestSkipped('Division by zero triggers a PHP warning before PHP 7.');
     }
     $vs = new qtype_calculated_variable_substituter(array('a' => 1, 'b' => 0), '.');
     $vs->calculate('{a} / {b}');
 }