Interface:

    apiver v1_0_0
    apiver changed v1_0_3
    apiver changed v1_1_0
    func parse_datetime(ref String s, ref writeonly Date d) -> bool

Implementation:

    func parse_datetime(ref String s, ref writeonly Date d) -> bool
    {
        parse_date(...)
        ...
        if apiver >= v1_1_0 {
             ...
        }
        ...
        bool bugcompat = apiver < v1_0_3
        ...
    }


Generates this code:

    type parse_datetime$apiver = enum (
        v1_0_0 = 0
        v1_0_3
        v1_1_0
    )

    func parse_datetime@v1_0_0(ref String s, ref writeonly Date d) -> bool
    {
        parse_datetime$internal(s, d, v1_0_0)
    }

    func parse_datetime@v1_0_3(ref String s, ref writeonly Date d) -> bool
    {
        parse_datetime$internal(s, d, v1_0_3)
    }

    func parse_datetime@v1_1_0(ref String s, ref writeonly Date d) -> bool
    {
        parse_datetime$internal(s, d, v1_1_0)
    }

    func parse_datetime(ref String s, ref writeonly Date d, enum parse_datetime$apiver ver) -> bool
    {
        parse_date(...)
        ...
        if apiver >= .v1_1_0 {
             ...
        }
        ...
        bool bugcompat = apiver < .v1_0_3
        ...
    }