Lisphp_Scope::listSymbols PHP Method

    public function listSymbols()
    {
        $symbols = array_keys($this->values);
        if (!$this->superscope) {
            return $symbols;
        }
        $symbols = array_merge($this->superscope->listSymbols(), $symbols);
        return array_unique($symbols);
    }

 function testListSymbols()
 {
     $this->assertEquals(array(), array_diff(array('abc', 'def', 'ghi'), $this->scope->listSymbols()));
     $scope = new Lisphp_Scope($this->scope);
     $scope->let('jkl', 123);
     $scope->let('abc', 456);
     $this->assertEquals(array(), array_diff(array('def', 'ghi', 'jkl', 'abc'), $scope->listSymbols()));
 }