reserve x,A,B,X,X9,Y,Y9,Z,V for set;

theorem
  X \/ Y \/ Z \/ V = X \/ (Y \/ Z \/ V)
proof
  X \/ Y \/ Z \/ V = X \/ Y \/ (Z \/ V) by Th4
    .= X \/ (Y \/ (Z \/ V)) by Th4
    .= X \/ (Y \/ Z \/ V) by Th4;
  hence thesis;
end;
