anchor.diff:      %% Wait for read
       eagain = read_nif(R, 3),
       0 = select_nif(R,?ERL_NIF_SELECT_READ,R,null,Ref),
  -    [] = flush(),
  +    [] = flush(0),
       ok = write_nif(W, <<"hej">>),
       [{select, R, Ref, ready_input}] = flush(),
       0 = select_nif(R,?ERL_NIF_SELECT_READ,R,self(),Ref2),
  
positive.code:     0 = select_nif(R,?ERL_NIF_SELECT_READ,R,null,Ref1),
      0 = select_nif(R,?ERL_NIF_SELECT_READ,R,self(),Ref2),
  
      [] = flush(),
      ok = write_nif(W, <<"hej">>),
      [{select, R, Ref2, ready_input}] = flush(),
      <<"hej">> = read_nif(R, 3),
  
negative.code:     %% Wait for write
      Written = write_full(W, $a),
      0 = select_nif(W,?ERL_NIF_SELECT_WRITE,W,self(),Ref),
      [] = flush(0),
      Written = read_nif(R,byte_size(Written)),
      [{select, W, Ref, ready_output}] = flush(),
  
  