theorem
  for f1 st f1 is circular & 1<=len f1 holds f1.1=f1.len f1
proof
  let f1;
  assume that
A1: f1 is circular and
A2: 1<=len f1;
A3: f1.1=f1/.1 by A2,FINSEQ_4:15;
  f1/.1=f1/.len f1 by A1;
  hence thesis by A2,A3,FINSEQ_4:15;
end;
