perms templates
  findPerms source
    @ set [1..$@perms::length];
    1 -> # !
    when <|..0> do VOID
    when <|$@perms::length..> do
      $@perms !
      $ - 1 -> # !
    when <|?($@($) matches <|$@perms::length~..>)> do
      @perms([$@perms::last, $..~$@perms::last]) set $@perms($..)...;
      @($) set $;
      $ - 1 -> #!
    otherwise
      @perms([$@($), $]) set $@perms([$, $@($)])...;
      @($) set $@($) + 1;
      $ + 1 -> # !
  end findPerms
  @ set [1..$];
  $findPerms !
end perms

alpha is ['ABCD'...];
[4 -> perms -> '$alpha($)...;' ] !