anchor.diff:          let mutable res = acc
           for kvp in firstEntries do
               res <- f kvp.Key kvp.Value res
  -            match x.GetRest(kvp.Key)  with
  +            match x.GetRest(kvp.Key) with
               | [] -> ()
               | rest -> 
                   for z in rest do
  
positive.code:     member x.Replace(y,z) = 
          firstEntries.[y] <- z
  
      member x.TryFind(y) = 
          let mutable res = Unchecked.defaultof<'Value>
          let ok = firstEntries.TryGetValue(y,&res)
          if ok then Some(res) else None
  
negative.code:         with get(y : 'Key) = 
              let mutable res = Unchecked.defaultof<'Value>
              let ok = firstEntries.TryGetValue(y,&res)
              if ok then res else raise (KeyNotFoundException("The item was not found in collection"))
          and set (y:'Key) (z:'Value) = 
              x.Replace(y,z)
  
  