theorem Th78:
  |-_IPC (p '&' (p => q)) => q
proof
  set X = {p '&' (p => q)};
A1: X |-_IPC p by Th73;
  X |-_IPC p => q by Th74; then
  X |-_IPC q by A1,Th27;
  hence thesis by Th54;
end;
