anchor.diff:  import 'universe/use.dart';
   import 'universe/world_impact.dart'
       show ImpactUseCase, WorldImpact, WorldImpactVisitorImpl;
  +import 'util/maplet.dart';
   import 'util/util.dart' show makeUnique;
   import 'world.dart' show KClosedWorld;
   
  
positive.code:         b = b._previous;
        }
      }
      while (imports.isNotEmpty) {
        result = result._add(imports.removeLast());
      }
      return result;
    }
  
negative.code:   }
  
    /// Links to other import sets in the lattice by adding one import.
    final Map<_DeferredImport, ImportSet> _transitions = Maplet();
  
    ImportSet.empty()
        : _import = null,
  