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 Th7:
  a /\ (b \/ a) = a
proof
  thus a /\ (b \/ a) = a /\ b \/ a /\ a by Th5
    .= a by Th6;
end;
