let p be Element of MC-wff ; :: thesis: (p 'or' p) => p in IPC-Taut
A1: p => p in IPC-Taut by Th17;
(p => p) => ((p => p) => ((p 'or' p) => p)) in IPC-Taut by Def14;
then (p => p) => ((p 'or' p) => p) in IPC-Taut by A1, Def14;
hence (p 'or' p) => p in IPC-Taut by A1, Def14; :: thesis: verum