theorem Th21:
  |:f1,f2:|.([a1,a2],[b1,b2]) = [f1.(a1,b1),f2.(a2,b2)]
proof
A1: dom |:f1,f2:| = [:[:D1,D2:],[:D1,D2:]:] by FUNCT_2:def 1;
  [[a1,a2],[b1,b2]] in [:[:D1,D2:],[:D1,D2:]:];
  hence thesis by A1,FUNCT_4:55;
end;
