code: public void setArg(Expression arg, int argNum)
            throws WrongNumberArgsException
    {
  
      // System.out.println("argNum: "+argNum);
      if (argNum == 0)
        super.setArg(arg, argNum);
      else if (1 == argNum)
      {
        m_arg1 = arg;
        arg.exprSetParent(this);
      }
      else
  		  reportWrongNumberArgs();
    }
docs: Set an argument expression for a function.  This method is called by the
  XPath compiler.
  
  @param arg non-null expression that represents the argument.
  @param argNum The argument number index.
  
  @throws WrongNumberArgsException If the argNum parameter is greater than 1.