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