anchor.diff:  
                   // deallocate component
                   correlations.Remove entityId |> ignore<bool>
  -                correlationsBack.Remove index |> ignore<bool>
   
                   // deallocate index for component
                   if index <> freeIndex
  
positive.code: 
              // allocate component
              correlations.Add (entityId, index)
              correlationsBack.Add (index, entityId)
              correlateds.Array.[index] <- comp
  
              // register correlation with ecs if needed
  
negative.code:     let mutable freeIndex = 0
      let freeList = HashSet<int> HashIdentity.Structural
      let correlations = dictPlus<Guid, int> HashIdentity.Structural []
      
      new (buffered, ecs) = SystemCorrelated (false, buffered, ecs)
      new (ecs) = SystemCorrelated (false, false, ecs)
  