theorem
  proj1_3(X /\ Y) c= proj1_3 X /\ proj1_3 Y
proof
  proj1_3(X /\ Y) c= proj1_3 X & proj1_3(X /\ Y) c= proj1_3 Y
       by Th10,XBOOLE_1:17;
 hence thesis by XBOOLE_1:19;
end;
