code: public void setArg(Expression arg, int argNum)
            throws WrongNumberArgsException
    {
      m_argVec.addElement(arg);
      arg.exprSetParent(this);
    }
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 beyond what
  is specified for this function.