Pinq\Expressions\Expression::hash PHP 메소드

    public final function hash()
    {
        return md5($this->compile());
    }

 public function createReference(O\Expression $expression, O\Expression $referencedExpression)
 {
     $this->expressionTypes[$expression->hash()] =& $this->expressionTypes[$referencedExpression->hash()];
 }