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

theorem Th58:
  (p => q) => (p => (q 'or' s)) in IPC-Taut
proof
A1: (q => (q 'or' s)) => ((p => q) => (p => (q 'or' s))) in IPC-Taut by Th22;
  q => (q 'or' s) in IPC-Taut by Def14;
  hence thesis by A1,Def14;
end;
