reserve T, X, Y for Subset of MC-wff;
reserve p, q, r, s for Element of MC-wff;

theorem Th60:
  ( p => q ) => ((p 'or' s) => (q 'or' s)) in IPC-Taut
proof
  set P = p 'or' s, Q = q 'or' s;
A1: (p => q) => (p => Q) in IPC-Taut by Th58;
  (p => Q) => ((s => Q) => (P => Q)) in IPC-Taut by Def14;
  then
A2: (s => Q) => ((p => Q) => (P => Q)) in IPC-Taut by Th23;
  s => Q in IPC-Taut by Def14;
  then (p => Q) => (P => Q) in IPC-Taut by A2,Def14;
  hence thesis by A1,Th26;
end;
