anchor.diff:        outputStream.add(transition.token);
         state = new State(rules, position).derive(ch);
       } on NoMatch catch(e){
  +      print(debug);
         _subscription.cancel();
         outputStream.addError(e);
       }
  
positive.code: part 'src/language.dart';
  part 'src/rules.dart';
  part 'src/builder.dart';
  
  class Token {
    final String value;
  
negative.code:         [this.matchStr='', this.lastMatch]);
  
    /// Find next state.
    /// TODO: I need to favor 'matchables' over 'exactMatch' if any exist.
    State derive(ch){
      if(rules.isEmpty) {
        throw new NoMatch(this);
      }
      if(lastMatch != null){
        return deriveLastMatch(ch);
  