reserve T, X, Y for Subset of HP-WFF;
reserve p, q, r, s for Element of HP-WFF;

theorem Th30:
  p => (p '&' p) in HP_TAUT
proof
A1: (p => (p => (p '&' p))) => (p => (p '&' p)) in HP_TAUT by Th27;
  p => (p => (p '&' p)) in HP_TAUT by Def10;
  hence thesis by A1,Def10;
end;
