theorem Th4:
  for D being non empty set, f being FinSequence of D st
    f just_once_values f/.len f holds f-:f/.len f = f
proof
  let D be non empty set, f be FinSequence of D;
  assume
A1: f just_once_values f/.len f;
  thus f-:f/.len f = f|(f/.len f..f) by FINSEQ_5:def 1
    .= f|len f by A1,Th1
    .= f by FINSEQ_1:58;
end;
