let p, q, s be Element of MC-wff ; (p '&' (q '&' s)) => (p '&' (s '&' q)) in IPC-Taut
set P = q '&' s;
set Q = s '&' q;
A1:
(q '&' s) => (s '&' q) in IPC-Taut
by Th46;
((q '&' s) => (s '&' q)) => ((p '&' (q '&' s)) => (p '&' (s '&' q))) in IPC-Taut
by Th49;
hence
(p '&' (q '&' s)) => (p '&' (s '&' q)) in IPC-Taut
by A1, Def14; verum