type key.

channel c.

const a: bitstring.
const b: key.

fun enc(channel, key): bitstring.
reduc forall c: channel, k: key; dec(enc(c,k),k) = c.

process
	in(choice[dec(a,b),c], x: bitstring); 0

(* EXPECTPV
RESULT Observational equivalence cannot be proved.
0.004s (user 0.004s + system 0.000s), max rss 8564K
END *)
