anchor.diff:  module Id =
       let run   (x:Id<_>) = x.getValue
       let map f (x:Id<_>) = Id (f x.getValue)
  -    let create x = Id (x)+    let create x = Id (x)
  
positive.code:             yield element
              notFirst := true}
  
      let inline splitBy keyMapper (source:_ seq) = seq {
          use e = source.GetEnumerator()
          if (e.MoveNext()) then
              let groupKey = ref (keyMapper e.Current)
              let values   = ref (ResizeArray())
              (!values).Add(e.Current)
              while (e.MoveNext()) do
                  let key = keyMapper e.Current
                  if !groupKey = key then (!values).Add(e.Current)
                  else
                      yield (!groupKey, !values)
                      groupKey := key
                      values   := ResizeArray()
                      (!values).Add(e.Current)
              yield (!groupKey, !values)}
  
  
  [<RequireQualifiedAccess>]
  module internal Error =
  
negative.code: namespace FsControl.Core
  
  open System.Collections.Generic
  
  module internal Prelude =
      let inline flip f x y = f y x
  