let p, q, s be Element of MC-wff ; :: thesis: ((p '&' q) '&' s) => (p '&' (q '&' s)) in IPC-Taut
A1: (p '&' (s '&' q)) => (p '&' (q '&' s)) in IPC-Taut by Lm15;
((p '&' q) '&' s) => (p '&' (s '&' q)) in IPC-Taut by Lm14;
hence ((p '&' q) '&' s) => (p '&' (q '&' s)) in IPC-Taut by A1, Th26; :: thesis: verum