code: static Node fuseExpressionIntoExpression(Node exp1, Node exp2) {
      if (exp2.isEmpty()) {
        return exp1;
      }
      Node comma = new Node(Token.COMMA, exp1);
      comma.useSourceInfoIfMissingFrom(exp2);
  
      // We can just join the new comma expression with another comma but
      // lets keep all the comma's in a straight line. That way we can use
      // tree comparison.
      if (exp2.isComma()) {
        Node leftMostChild = exp2;
        while (leftMostChild.isComma()) {
          leftMostChild = leftMostChild.getFirstChild();
        }
        Node parent = leftMostChild.getParent();
        comma.addChildToBack(leftMostChild.detach());
        parent.addChildToFront(comma);
        return exp2;
      } else {
        comma.addChildToBack(exp2);
        return comma;
      }
    }
docs: exp1, exp1