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

theorem Th4:
  for s be FinSequence st 1 <= len s
  holds len (Op-LeftShift(s)) = len s
  proof
    let s be FinSequence;
    assume A1: 1 <= len s;
    A2: len <* s.1 *> = 1 by FINSEQ_1:40;
    len (s /^1) = (len s) - 1 by A1,RFINSEQ:def 1;
    then
    len ((s /^1) ^ <* s.1 *>) = ((len s) - 1) + 1 by A2,FINSEQ_1:22
    .= (len s);
    hence thesis;
  end;
