templates perms
  templates findPerms
    when <$@perms::length..> do $@perms !
    otherwise
      def index: $;
      $index..$@perms::length
      -> \(
          @perms([$, $index]): $@perms([$index, $])...;
          $index + 1 -> findPerms !
      \) !
      @perms([last, $index..last-1]): $@perms($index..last)...;
  end findPerms
  @: [1..$];
  1 -> findPerms !
end perms

def alpha: ['ABCD'...];
[4 -> perms -> '$alpha($)...;' ] -> !OUT::write