anchor.diff:  %%  switch list to a #b_br{}.
   
   -spec normalize(b_set() | terminator()) ->
  -                       b_set() | terminator().
  +          b_set() | terminator().
   
   normalize(#b_set{op={bif,Bif},args=Args}=Set) ->
       case {is_commutative(Bif),Args} of
  
positive.code:     {Blocks, Acc}.
  
  -spec mapfold_instrs(Fun, Labels, Acc0, Blocks0) -> {Blocks,Acc} when
        Fun :: fun((b_blk()|terminator(), any()) -> any()),
        Labels :: [label()],
        Acc0 :: any(),
        Acc :: any(),
  
negative.code:     mapfold_instrs_1(Labels, Fun, Blocks, Acc0).
  
  -spec flatmapfold_instrs(Fun, Labels, Acc0, Blocks0) -> {Blocks,Acc} when
        Fun :: fun((b_set()|terminator(), any()) -> any()),
        Labels :: [label()],
        Acc0 :: any(),
        Acc :: any(),
  