Derive a custom Validate from an existing one

First let me apologize for creating an issue for a simple question, but I haven't found another channel.

So, let's say I would like to refine `scala.concurrent.duration.FiniteDuration`, e.g. with `Positive`. My first idea was to simply use `Positive` and (contra)map it over a function extracting the `length` from the `FiniteDuration` which is a `Long`. But I haven't found a way to do that, i.e. no public (contra)map method on `Validate`.

Any help would be highly appreciated.