let p, q, s be Element of MC-wff ; :: thesis: (p 'or' (q 'or' s)) => ((p 'or' q) 'or' s) in IPC-Taut
A1: (s 'or' (p 'or' q)) => ((p 'or' q) 'or' s) in IPC-Taut by Th56;
(q 'or' s) => (s 'or' q) in IPC-Taut by Th56;
then A2: (p 'or' (q 'or' s)) => (p 'or' (s 'or' q)) in IPC-Taut by Th63;
(p 'or' (s 'or' q)) => (s 'or' (p 'or' q)) in IPC-Taut by Th64;
then (p 'or' (q 'or' s)) => (s 'or' (p 'or' q)) in IPC-Taut by A2, Th26;
hence (p 'or' (q 'or' s)) => ((p 'or' q) 'or' s) in IPC-Taut by A1, Th26; :: thesis: verum