def name(self):
    """
    Save name which can be used for alphabetic sorting and can be turned
    into a kwarg.
    """
    base_str = 'd{}{}_'.format(self.derivative_count if
                               self.derivative_count > 1 else '', self.expr)
    for var, count in self.variable_count:
        base_str += 'd{}{}'.format(var,  count if count > 1 else '')
    return base_str
Save name which can be used for alphabetic sorting and can be turned
    into a kwarg.