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diff --git a/css/lib/lex_buffer.ml b/css/lib/lex_buffer.ml
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+(* Based on
+ * https://github.com/smolkaj/ocaml-parsing/blob/master/src/LexBuffer.ml *)
+
+(** A custom lexbuffer that automatically keeps track of the source location.
+    This module is a thin wrapper arounds sedlexing's default buffer, which does
+    not provide this functionality. *)
+
+type t = {
+  buf : Sedlexing.lexbuf;
+  mutable pos : Lexing.position;
+  mutable pos_mark : Lexing.position;
+  mutable last_char : int option;
+  mutable last_char_mark : int option;
+}
+(** the lex buffer type *)
+
+let of_sedlex ?(file = "<n/a>") ?pos buf =
+  let pos =
+    match pos with
+    | None ->
+        {
+          Lexing.pos_fname = file;
+          pos_lnum = 1;
+          (* line number *)
+          pos_bol = 0;
+          (* offset of beginning of current line *)
+          pos_cnum = 0 (* total offset *);
+        }
+    | Some p -> p
+  in
+  { buf; pos; pos_mark = pos; last_char = None; last_char_mark = None }
+
+let of_ascii_string ?pos s = of_sedlex ?pos (Sedlexing.Latin1.from_string s)
+
+let of_ascii_file file =
+  let chan = open_in file in
+  of_sedlex ~file (Sedlexing.Latin1.from_channel chan)
+
+(** The next four functions are used by sedlex internally.
+    See https://www.lexifi.com/sedlex/libdoc/Sedlexing.html.  *)
+let mark lexbuf p =
+  lexbuf.pos_mark <- lexbuf.pos;
+  lexbuf.last_char_mark <- lexbuf.last_char;
+  Sedlexing.mark lexbuf.buf p
+
+let backtrack lexbuf =
+  lexbuf.pos <- lexbuf.pos_mark;
+  lexbuf.last_char <- lexbuf.last_char_mark;
+  Sedlexing.backtrack lexbuf.buf
+
+let start lexbuf =
+  lexbuf.pos_mark <- lexbuf.pos;
+  lexbuf.last_char_mark <- lexbuf.last_char;
+  Sedlexing.start lexbuf.buf
+
+(** location of next character *)
+let next_loc lexbuf = { lexbuf.pos with pos_cnum = lexbuf.pos.pos_cnum + 1 }
+
+let cr = Char.code '\r'
+
+(** next character *)
+let next lexbuf =
+  let c = Sedlexing.next lexbuf.buf in
+  let pos = next_loc lexbuf in
+  let ch =
+    match c with
+    | None -> None
+    | Some c -> ( try Some (Uchar.to_char c) with Invalid_argument _ -> None )
+  in
+  ( match ch with
+  | Some '\r' ->
+      lexbuf.pos <-
+        { pos with pos_bol = pos.pos_cnum - 1; pos_lnum = pos.pos_lnum + 1 }
+  | Some '\n' when not (lexbuf.last_char = Some cr) ->
+      lexbuf.pos <-
+        { pos with pos_bol = pos.pos_cnum - 1; pos_lnum = pos.pos_lnum + 1 }
+  | Some '\n' -> ()
+  | _ -> lexbuf.pos <- pos );
+  ( match c with
+  | None -> lexbuf.last_char <- None
+  | Some c -> lexbuf.last_char <- Some (Uchar.to_int c) );
+  c
+
+let raw lexbuf = Sedlexing.lexeme lexbuf.buf
+
+let latin1 ?(skip = 0) ?(drop = 0) lexbuf =
+  let len = Sedlexing.lexeme_length lexbuf.buf - skip - drop in
+  Sedlexing.Latin1.sub_lexeme lexbuf.buf skip len
+
+let utf8 ?(skip = 0) ?(drop = 0) lexbuf =
+  let len = Sedlexing.lexeme_length lexbuf.buf - skip - drop in
+  Sedlexing.Utf8.sub_lexeme lexbuf.buf skip len
+
+let container_lnum_ref = ref 0
+
+let fix_loc loc =
+  let fix_pos pos =
+    (* It looks like lex_buffer.ml returns a position with 2 extra
+     * chars for parsed lines after the first one. Bug? *)
+    let pos_cnum =
+      if pos.Lexing.pos_lnum > !container_lnum_ref then pos.Lexing.pos_cnum - 2
+      else pos.Lexing.pos_cnum
+    in
+    { pos with Lexing.pos_cnum }
+  in
+  let loc_start = fix_pos loc.Location.loc_start in
+  let loc_end = fix_pos loc.Location.loc_end in
+  { loc with Location.loc_start; loc_end }
+
+let make_loc ?(loc_ghost = false) start_pos end_pos : Location.t =
+  { Location.loc_start = start_pos; loc_end = end_pos; loc_ghost }
+
+let make_loc_and_fix ?(loc_ghost = false) start_pos end_pos : Location.t =
+  make_loc ~loc_ghost start_pos end_pos |> fix_loc