theorem Th14:
  i in Seg n implies a.i is Element of k-SD
proof
A1: len a = n by CARD_1:def 7;
  assume i in Seg n;
  then i in dom a by A1,FINSEQ_1:def 3;
  then
A2: a.i in rng a by FUNCT_1:def 3;
  rng a c= k-SD by FINSEQ_1:def 4;
  hence thesis by A2;
end;
