let p, q, s be Element of MC-wff ; :: thesis: (p '&' (q '&' s)) => ((s '&' q) '&' p) in IPC-Taut
A1: (p '&' (s '&' q)) => ((s '&' q) '&' p) in IPC-Taut by Th46;
(p '&' (q '&' s)) => (p '&' (s '&' q)) in IPC-Taut by Lm15;
hence (p '&' (q '&' s)) => ((s '&' q) '&' p) in IPC-Taut by A1, Th26; :: thesis: verum