let subbitstring (data, off, len) off' len' =
let off = off + off' in
- if len < off' + len' then invalid_arg "subbitstring";
+ if off' < 0 || len' < 0 || off' > len - len' then invalid_arg "subbitstring";
(data, off, len')
let dropbits n (data, off, len) =