anchor.diff:  
           member this.Iter fn =
               let system = this.Ecs.IndexSystem<'c, SystemCorrelated<'c, 'w>> ()
  -            for kvp in this.Cache do
  -                let entityId = kvp.Key
  +            for entityId in this.Cache do
                   fn (system.IndexCorrelated entityId)
   
       type Query<'c, 'c2, 'w when
  
positive.code: /// An ECS query.
  and [<AbstractClass>] 'w Query (filter : (string HashSet -> bool), ecs : 'w Ecs) =
  
      let cache = dictPlus<Guid, 'w EntityRef> HashIdentity.Structural []
  
      abstract Correlation : string HashSet
      member this.Ecs = ecs
  
negative.code:             let system4 = this.Ecs.IndexSystem<'c4, SystemCorrelated<'c4, 'w>> ()
              let system5 = this.Ecs.IndexSystem<'c5, SystemCorrelated<'c5, 'w>> ()
              let system6 = this.Ecs.IndexSystem<'c6, SystemCorrelated<'c6, 'w>> ()
              for entityId in this.Cache do
                  fn
                      (system.IndexCorrelated entityId)
                      (system2.IndexCorrelated entityId)
  