anchor.diff:      dom_intersection(Doms, Numbering).
   
   -spec fold_instrs(Fun, Labels, Acc0, Blocks) -> any() when
  -      Fun :: fun((b_blk()|terminator(), any()) -> any()),
  +      Fun :: fun((b_set()|terminator(), any()) -> any()),
         Labels :: [label()],
         Acc0 :: any(),
         Blocks :: block_map().
  
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(),
  