If I write a nice, simple use, like:

myPotato = map coerce

or even

myPotato = Data.OldList.map coerce

the "map/coerce" rule does not fire.

It looks like there are two things getting in the way:

1. The map rule fires first, turning map into a build/foldr form, which map/coerce does not recognize.
2. Even if map gets written back, coerce has been expanded into something the rule doesn't recognize.

So we end up with something that looks like

myPotato =
  \ (@ a_are)
    (@ b_arf)
    ($dCoercible_arz :: Coercible a_are b_arf)
    (lst_aqx :: [a_are]) ->
    map
      @ a_are
      @ b_arf
      (\ (tpl_B2 [OS=ProbOneShot] :: a_are) ->
         case $dCoercible_arz
         of _ [Occ=Dead] { GHC.Types.MkCoercible tpl1_B3 ->
         tpl_B2 `cast` (tpl1_B3 :: a_are ~R# b_arf)
         })
      lst_aqx