function shrinkBless(shrink) {
  shrink.smap = shrinkProtoIsoMap;
  return shrink;
}
- `shrink.bless(f: a -> [a]): shrink a`

      Bless function with `.smap` property.

  - `.smap(f: a -> b, g: b -> a): shrink b`

      Transform `shrink a` into `shrink b`. For example:

      ```js
      positiveIntegersShrink = nat.shrink.smap(
        function (x) { return x + 1; },
        function (x) { return x - 1; });
      ```