anchor.diff:  tryFind(K, M) ->
       maps:get(K, M, undefined).
   
  +remove(K, M) ->
  +    maps:remove(K, M).
  +
  
positive.code: -module('Microsoft.FSharp.Collections.MapModule').
  -compile(export_all).
  
  
  add(K, V, M) ->
  
negative.code:                       F(Acc, K, V)
                end, S, M).
  
  isEmpty(#{}) -> true;
  isEmpty(_) -> false.
  
  