anchor.diff:  type identifier_info = {
       identifier:either<bv, fv>;
       identifier_ty:typ;
  +    identifier_range:Range.range;
   }
   let rec insert_col_info col info col_infos =
       match col_infos with 
  
positive.code:       | Some cols -> 
          match find_nearest_preceding_col_info col !cols with
          | None -> None
          | Some ci -> Some ci//(info_as_string ci)
  let insert_bv bv ty = insert_identifier_info (Inl bv) ty (FStar.Syntax.Syntax.range_of_bv bv)
  let insert_fv fv ty = insert_identifier_info (Inr fv) ty (FStar.Syntax.Syntax.range_of_fv fv)
  
negative.code:     BU.imap<ref<col_info>>
  type file_info = 
      BU.smap<row_info>
  let mk_info id ty range = {
      identifier=id;
      identifier_ty=ty;
      identifier_range=range
  }
  let file_info_table : file_info = BU.smap_create 50 //50 files
  open FStar.Range
  let insert_identifier_info id ty range =
      let use_range = {range with def_range=range.use_range} in //key the lookup table from the use range
      let info = mk_info id ty use_range in
      let fn = Range.file_of_range use_range in
      let start = Range.start_of_range use_range in 
      let row, col = Range.line_of_pos start, Range.col_of_pos start in
  