i: Int = 2 - 1
assert i in Nat
i: Nat
assert i in Str # ERR, i cannot be of type Str

f(opt_i: Int or NoneType) =
    if opt_i != None, do:
        if opt_i >= 0, do:
            log opt_i.times!
        log opt_i + 1 # OK
        log opt_i.times! # ERR
    if isinstance(opt_i, Int), do:
        log opt_i + 1 # OK
    log opt_i + 1 # ERR

f(1)