theorem Th124:
  for f being FinSequence, k being Nat, p being set holds
    (<*p*>^f)|(k+1) = <*p*>^(f|k)
proof
  let f be FinSequence, k be Nat, p be set;
  thus (<*p*>^f)|(k+1) = (<*p*>^f)|Seg(k+len<*p*>) by FINSEQ_1:39
    .= <*p*>^(f|k) by Th14;
end;
