Insert an arg to the current expression.

def InsertArg(self, string="", **_):
    """Insert an arg to the current expression."""
    if self.state == "LIST_ARG":
      self.list_args.append(string)
    elif self.current_expression.AddArg(string):
      # This expression is complete
      self.stack.append(self.current_expression)
      self.current_expression = self.expression_cls()
      # We go to the BINARY state, to find if there's an AND or OR operator
      return "BINARY"