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

theorem Th2:
  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 \ (X \ Y) by XBOOLE_1:48;
  hence thesis;
end;
