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

theorem Th30: :: A Hilbert axiom
  (p => (p => q)) => (p => q) in IPC-Taut
proof
  (p => (p => q)) => ((p => p) => (p => q)) in IPC-Taut by Def14;
  then
A1: (p => p) => ((p => (p => q)) => (p => q)) in IPC-Taut by Th23;
  p => p in IPC-Taut by Th17;
  hence thesis by A1,Def14;
end;
