reserve x,y for object,X,Y,A,B,C,M for set;
reserve P,Q,R,R1,R2 for Relation;
reserve X,X1,X2 for Subset of A;
reserve Y for Subset of B;
reserve R,R1,R2 for Subset of [:A,B:];
reserve FR for Subset-Family of [:A,B:];

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;
