theorem
  len F = 1 implies Sum(F) = F.1
proof
  assume
A1: len F = 1;
  then dom F = {1} by FINSEQ_1:2,def 3;
  then 1 in dom F by TARSKI:def 1;
  then
A2: F.1 in rng F by FUNCT_1:def 3;
  rng F c= the carrier of V by FINSEQ_1:def 4;
  then reconsider v = F.1 as Element of V by A2;
  F = <* v *> by A1,FINSEQ_1:40;
  hence thesis by Lm6;
end;
