let p, q, s be Element of MC-wff ; :: thesis: (p '&' (q '&' s)) => ((p '&' s) '&' q) in IPC-Taut
A1: ((q '&' s) '&' p) => ((p '&' s) '&' q) in IPC-Taut by Lm13;
(p '&' (q '&' s)) => ((q '&' s) '&' p) in IPC-Taut by Lm18;
hence (p '&' (q '&' s)) => ((p '&' s) '&' q) in IPC-Taut by A1, Th26; :: thesis: verum