Fixed a bug in the restriction semantics: Actions with '!' or '?' were always forked into such, which caused restrict sets to not match suffix-less actions.
Added Set.toString.
Fixed some bugs in matching and printing restriction sets:

  - Actions with the same suffix were considered matching.
  - When printing restriction actions, the last character was always cut off, which is not valid any more since we also include the suffix-less names of all actions.