Type Guard experimentation of Set

xset.ts

interface XSet<X0> {
    has<X1 extends X0> (x: X1): this is XSetWith<X1, X0>
}

interface XSetWith<X1 extends X0, X0> extends XSet<X0> {
    has<X2 extends X0> (x: X2): this is XSetWith<X2, X1>
    has<X2 extends X1> (x: X2): true
}

declare const xset: XSet<number>

if (xset.has(2)) {
    const f = xset.has(2) // true
    const g = xset.has(3) // boolean

    if (xset.has(7)) {
        const ff = xset.has(2) // true
        const hh = xset.has(7) // true
        const ii = xset.has(8) // boolean
    }
}