code: public void addArgument(AstNode arg) {
          assertNotNull(arg);
          if (arguments == null) {
              arguments = new ArrayList<AstNode>();
          }
          arguments.add(arg);
          arg.setParent(this);
      }
docs: Adds an argument to the list, and sets its parent to this node.
  @param arg the argument node to add to the list
  @throws IllegalArgumentException} if arg is {@code null}