anchor.diff:  %%  a binary. MustSave is true if the function may pass the match
   %%  context to the bs_context_to_binary instruction (in which case
   %%  the current position in the binary must have saved into the
  -%%  start position using "bs_save_2 Ctx start".
  +%%  start position using "bs_save_2 Ctx start").
   
   btb_index(Fs) ->
       btb_index_1(Fs, []).
   
   btb_index_1([{function,_,_,Entry,Is0}|Fs], Acc0) ->
  -    [{label,Entry}|Is] =
  -	dropwhile(fun({label,L}) when L =:= Entry -> false;
  -		     (_) -> true
  -		  end, Is0),
  +    Is = drop_to_label(Is0, Entry),
       Acc = btb_index_2(Is, Entry, false, Acc0),
       btb_index_1(Fs, Acc);
   btb_index_1([], Acc) -> gb_trees:from_orddict(sort(Acc)).
  
positive.code:     catch
  	throw:none -> Acc
      end.
  
  btb_index_find_start_match([{test,_,{f,F},_},{bs_context_to_binary,_}|Is]) ->
      btb_index_find_label(Is, F);
  
negative.code: -module(beam_bsm).
  -export([module/2,format_error/1]).
  
  -import(lists, [member/2,foldl/3,reverse/1,sort/1,all/2]).
  
  %%%
  %%% We optimize bit syntax matching where the tail end of a binary is
  