code: public void setArg(Expression arg, int argNum)
            throws WrongNumberArgsException
    {
  
      if (0 == argNum)
      {
        m_arg0 = 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 0.