'Adds a new arg to this expression.
Args:
arg: The argument to add (string).
Returns:
True if this arg is the last arg, False otherwise.
Raises:
ParseError: If there are too many args.'

def AddArg(self, arg):

    self.args.append(arg)
    if (len(self.args) > self.number_of_args):
        raise ParseError('Too    many    args    for    this    expression.')
    elif (len(self.args) == self.number_of_args):
        return True
    return False
