fn ret(s string) string {
    return s
}

fn raise() ?string {
    return none
}

fn xx_prop() string {
    s := ret(raise()?)
    return s
}

struct St {
mut:
    z f64
}

fn (mut s St) raise() !f64 {
    return error('some error')
}

fn retf(f f64) f64 {
    return f
}

fn (mut s St) aa_propagate() {
    f := retf(s.raise()!)
    s.z = 7.5
    println(f)
}