theorem Th1:
  for f be FinSequence, i be Nat st i in dom f holds len Del(f,i) = len f -' 1
proof
  let f be FinSequence, i be Nat;
  assume i in dom f;
  then ex m be Nat st len f = m + 1 & len Del(f,i) = m by FINSEQ_3:104;
  hence thesis by NAT_D:34;
end;
