Ensure that our expression string has variables of the form x_0, x_1,
        ... x_(N - 1), where N is the length of our inputs.

def _validate(self):
        """
        Ensure that our expression string has variables of the form x_0, x_1,
        ... x_(N - 1), where N is the length of our inputs.
        """
        variable_names, _unused = getExprNames(self._expr, {})
        expr_indices = []
        for name in variable_names:
            if name == 'inf':
                continue
            match = _VARIABLE_NAME_RE.match(name)
            if not match:
                raise ValueError("%r is not a valid variable name" % name)
            expr_indices.append(int(match.group(2)))

        expr_indices.sort()
        expected_indices = list(range(len(self.inputs)))
        if expr_indices != expected_indices:
            raise ValueError(
                "Expected %s for variable indices, but got %s" % (
                    expected_indices, expr_indices,
                )
            )
        super(NumericalExpression, self)._validate()