expr: term (('+' | '-') term)*

def expr(self):
        """
        expr: term (('+' | '-') term)*
        """
        node = self.term()

        while self.token.nature in (Nature.PLUS, Nature.MINUS):
            token = self.token
            if token.nature == Nature.PLUS:
                self._process(Nature.PLUS)
            elif token.nature == Nature.MINUS:
                self._process(Nature.MINUS)
            else:
                self._error()

            node = BinaryOperation(left=node, op=token, right=self.term())

        return node