public function makeNode($leftOperand, $rightOperand)
    {
        $leftOperand = $this->sanitize($leftOperand);
        $rightOperand = $this->sanitize($rightOperand);

        $node = $this->numericTerms($leftOperand, $rightOperand);
        if ($node) return $node;

        return new ExpressionNode($leftOperand, '+', $rightOperand);
    }