Compute new expression strings and a new inputs tuple for combining
        self and other with a binary operator.

def build_binary_op(self, op, other):
        """
        Compute new expression strings and a new inputs tuple for combining
        self and other with a binary operator.
        """
        if isinstance(other, NumericalExpression):
            self_expr, other_expr, new_inputs = self._merge_expressions(other)
        elif isinstance(other, Term):
            self_expr = self._expr
            new_inputs, other_idx = _ensure_element(self.inputs, other)
            other_expr = "x_%d" % other_idx
        elif isinstance(other, Number):
            self_expr = self._expr
            other_expr = str(other)
            new_inputs = self.inputs
        else:
            raise BadBinaryOperator(op, other)
        return self_expr, other_expr, new_inputs