If a class Child inherits from Parent, how to implement Child.some_method if Parent.some_method() returns Parent instance ?

[metal]

On 10月30日, 上午8时03分, metal <metal... at gmail.com> wrote:
> The actual situation is I'm coding with a immuable set-like datatype
> XSet which supports XSet(['al']) & XSet(['ah'] = XSet(['ax'] if I
> declare ax is consists of al and ah
>

A typo,  XSet(['al']) | XSet(['ah'] = XSet(['ax']