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

theorem Th61:
  p => q in IPC-Taut implies (p 'or' s) => (q 'or' s) in IPC-Taut
proof
  assume
A1: p => q in IPC-Taut;
  ( p => q ) => ((p 'or' s) => (q 'or' s)) in IPC-Taut by Th60;
  hence thesis by A1,Def14;
end;
