theorem
  len F = 2 & v1 = F.1 & v2 = F.2 implies Sum(F) = v1 + v2
proof
  assume len F = 2 & v1 = F.1 & v2 = F.2;
  then F = <* v1,v2 *> by FINSEQ_1:44;
  hence thesis by Th45;
end;
