free c:channel.
free k:bitstring [private].
free d_P:channel [private].
free d_Q:channel [private].

fun senc(nat,bitstring):bitstring.
reduc forall K:bitstring,M:nat; sdec(senc(M,K),K) = M.

event CheckNat(nat).

query i:nat; event(CheckNat(i)) ==> is_nat(i).

let P =
  in(c,x:bitstring);
  in(d_P,(i:nat,j:nat));
  let j':nat = sdec(x,k) in
  event CheckNat(i);
  event CheckNat(j);
  if j' > j
  then out(d_P,(i+1,j'))
  else out(d_P,(i,j)).

let Q =
  in(d_Q,i:nat);
  out(c,senc(i,k));
  out(d_Q,i+1).

process
  out(d_P,(0,0)) | out(d_Q,0) | ! P | ! Q
