The team intend to modify the existing \<thing>_case:nnn functions,
renaming the last argument as 'F'. This reflects the fact that this
final argument is used only when the test case (<thing>-dependent) is
logically false, and follows the approach used elsewhere. At the same
time, we intend to introduce \<thing>_case:nn, where there is no false
branch (i.e. where 'do nothing' is the appropriate action).