theorem Th49:
  X |-_IPC p => p
proof
A1: X |-_IPC p => (p => p) by INTPRO_1:1;
A2: X |-_IPC p => ((p => p) => p) by INTPRO_1:1;
  X |-_IPC (p => ((p => p) => p)) => ((p => (p => p)) => (p => p))
    by INTPRO_1:2;
  then X |-_IPC (p => (p => p)) => (p => p) by A2,Th27;
  hence thesis by A1,Th27;
end;
