reserve X,Y,x for set;
reserve A for non empty preBoolean set;

theorem Th3:
  X is Element of A & Y is Element of A implies X \+\ Y is Element of A
proof
  assume X is Element of A & Y is Element of A;
  then reconsider X,Y as Element of A;
  X \+\ Y = (X \ Y) \/ (Y \ X);
  hence thesis;
end;
