reserve D for non empty set;
reserve s for FinSequence of D;
reserve m,n for Element of NAT;

theorem Th5:
  1 <= len s implies
  Op-LeftShift(s) is FinSequence of D &
  len (Op-LeftShift(s)) = len s
  proof
    assume A1: 1 <= len s;
    then 1 in Seg (len s);
    then 1 in dom s by FINSEQ_1:def 3;
    then s.1 is Element of D by FINSEQ_2:11;
    then s /^1 is FinSequence of D & <* s.1 *> is FinSequence of D
    by FINSEQ_1:74;
    hence thesis by Th4,A1,FINSEQ_1:75;
  end;
