theorem
  p in rng f implies (Rotate(f,p))/.len(f:-p) = f/.len f
proof
A1: 1 <= len (f:-p) by Th6;
  assume
A2: p in rng f;
  then p..f <= len f by FINSEQ_4:21;
  then reconsider x = len f - p..f as Element of NAT by INT_1:5;
  len (f:-p) -' 1 + p..f = x + 1 -' 1 + p..f by A2,FINSEQ_5:50
    .= len f - p..f + p..f by NAT_D:34
    .= len f;
  hence thesis by A1,A2,Th9;
end;
