free a,b: bitstring [private].

channel c.

query attacker(a); attacker(b).


let P(x:bitstring)=
   sync 1;
   out(c,x).

process
    P(a) [sync: tag prefix T1] | P(b) [sync:tag prefix T1]

(* EXPECTPV
Error: Same tag T1_@_21 used again at the same synchronization level 1 in parallel process.
xtime: error in child process (status : 2)
END *)
