anchor.diff:    return(enacl:sign_keypair()).
   
   sign_keypair() ->
  -  fault(sign_keypair_bad(), sign_keypair_good()).
  +  z_fault(sign_keypair_bad(), sign_keypair_good()).
   
   sign_keypair_public_valid(#{ public := Public })
     when is_binary(Public) ->
  
positive.code: 
  prop_sign() ->
      ?FORALL({Msg, KeyPair},
            {fault_rate(1, 40, g_iodata()),
             fault_rate(1, 40, sign_keypair())},
        begin
          case v_iodata(Msg) andalso sign_keypair_secret_valid(KeyPair) of
            true ->
  
negative.code: 
  %% Generator of natural numbers
  g_nat() ->
      z_fault(g_nat_bad(), nat()).
  
  g_nat_bad() ->
      oneof([
  