final public function walk(Expression $expression = null)
    {
        if ($expression === null) {
            return null;
        }

        return $this->doWalk($expression);
    }
final public function walk(Expression $expression = null)
    {
        if ($expression === null) {
            return null;
        }

        return $this->doWalk($expression);
    }
final public function walk ( Expression $ expression = null ) { if ( $ expression === null ) { return null ; } return $ this -> doWalk ( $ expression ) ; }
Walks the expression tree and returns the updated expression tree.

@param Expression $expression

@return Expression|null
Walks the expression tree and returns the updated expression tree .