theorem :: (2)
  R.:^(X1\/X2) = (R.:^X1) /\ (R.:^X2)
proof
  R.:^(X1\/X2) = Intersect(.:R.:({_{X1}_} \/ {_{X2}_})) by Th3
    .= Intersect(.:R.:{_{X1}_} \/ (.:R.:{_{X2}_})) by RELAT_1:120
    .= R.:^X1 /\ (R.:^X2) by MSSUBFAM:8;
  hence thesis;
end;
