reserve p,q,x,x1,x2,y,y1,y2,z,z1,z2 for set;
reserve A,B,V,X,X1,X2,Y,Y1,Y2,Z for set;
reserve C,C1,C2,D,D1,D2 for non empty set;

theorem Th50:
  for f,g being Function st dom f = X & dom g = X holds dom <:f,g :> = X
proof
  let f,g be Function;
  dom <:f,g:> = dom f /\ dom g by Def7;
  hence thesis;
end;
