theorem
  n+1 = len f implies (f/^n) = <*f.(n+1)*>
  proof
    assume n+1 = len f; then
    (f|n)^<*f.(n+1)*> = f by RFINSEQ:7
    .= (f|n)^(f/^n) by RFINSEQ:8;
    hence thesis by FINSEQ_1:33;
  end;
