theorem Th1:
  for D being non empty set, f being FinSequence of D st
    f just_once_values f/.len f holds f/.len f..f = len f
proof
  let D be non empty set, f be FinSequence of D;
  assume
A1: f just_once_values f/.len f;
  then reconsider f9 = f as non empty FinSequence of D by FINSEQ_4:5,RELAT_1:38
;
  f/.len f..f + 1 = f/.len f..f + ((Rev f9)/.1)..Rev f9 by Th43
    .= f/.len f..f + f/.len f..Rev f by FINSEQ_5:65
    .= len f + 1 by A1,Th37;
  hence thesis;
end;
