free c: channel.
free m,n: bitstring.

set swapping = "@_20 -> @_21".

process
    (
        out(c,m);
        sync 1;
        out(c,choice[m,n])
    )|(
        sync 1;
        out(c,choice[n,m])
    )

(* EXPECTPV
RESULT Observational equivalence is true.
0.016s (user 0.016s + system 0.000s), max rss 8780K
END *)
