theorem Th77:
  |-_IPC ((p => r) '&' (q => r)) => ((p 'or' q) => r)
proof
  set X = {(p => r) '&' (q => r)};
A1: X |-_IPC p => r by Th73;
A2: X |-_IPC q => r by Th74;
  X |-_IPC (p => r) => ((q => r) => ((p 'or' q) => r)) by Th25; then
  X |-_IPC (q => r) => ((p 'or' q) => r) by A1,Th27; then
  X |-_IPC (p 'or' q) => r by A2,Th27;
  hence thesis by Th54;
end;
