'Insert an arg to the current expression.'

def InsertArg(self, string='', **_):

    if (self.state == 'LIST_ARG'):
        self.list_args.append(string)
    elif self.current_expression.AddArg(string):
        self.stack.append(self.current_expression)
        self.current_expression = self.expression_cls()
        return 'BINARY'
