theorem
  len F = 3 & v1 = F.1 & v2 = F.2 & v = F.3 implies Sum(F) = v1 + v2 + v
proof
  assume len F = 3 & v1 = F.1 & v2 = F.2 & v = F.3;
  then F = <* v1,v2,v *> by FINSEQ_1:45;
  hence thesis by Th46;
end;
