anchor.diff:    }
     
     void _subsetConstraint(TypeIdentifier a, TypeIdentifier b) {
  +    
       if (a != b)
         foreach(a).update((AbstractType type) => types.put(b, type));
     }
  
positive.code: import 'types.dart';
  import 'util.dart';
  import 'dart:collection';
  
  class ConstraintAnalysis {
    TypeMap typeMap;
  
negative.code:   }
    
    bool remove(void func(AbstractType)){
      return _event_listeners.remove(func);
    }
    
  