anchor.diff:        case State.separator:
           state = State.next;
           current = iterator.current;
  -        break;
  +        return true;
         case State.after:
           state = State.complete;
           current = null;
  -        break;
  +        return false;
         case State.complete:
           return false;
       }
  
positive.code:       case State.before:
          state = State.next;
          current = iterator.current;
          break;
        case State.next:
          if (iterator.moveNext()) {
            state = State.separator;
  
negative.code: 
  import 'dart:collection';
  
  /// Function type to build elements.
  typedef Builder<E> = E Function();
  
  /// States of the iterator statemachine.
  