theorem Th17:
  F.:(i|->d,i|->d9) = i |-> (F.(d,d9))
proof
  [d,d9] in [:D,D9:] by ZFMISC_1:def 2;
  then [d,d9] in dom F by FUNCT_2:def 1;
  hence thesis by Th7;
end;
