theorem Th129:
 for D being set holds product (i |-> D) = i-tuples_on D
proof let D be set;
  thus product (i |-> D) = product (Seg i --> D)
    .= Funcs(Seg i,D) by CARD_3:11
    .= i-tuples_on D by FINSEQ_2:93;
end;
