reserve A, B for non empty preBoolean set,
  x, y for Element of [:A,B:];
reserve X for set,
  a,b,c for Element of [:A,B:];

theorem Th6:
  a \/ (b /\ a) = a
proof
A1: (a \/ (b /\ a))`2 = a`2 \/ (b /\ a)`2
    .= a`2 \/ b`2 /\ a`2
    .= a`2 by XBOOLE_1:22;
  (a \/ (b /\ a))`1 = a`1 \/ (b /\ a)`1
    .= a`1 \/ b`1 /\ a`1
    .= a`1 by XBOOLE_1:22;
  hence thesis by A1,DOMAIN_1:2;
end;
