anchor.diff:  
   -spec empty() -> tree().
   empty() ->
  -    %% PLACEHOLDER
       #contract_tree{}.
   
   -spec insert_contract(aect_contracts:contract(), tree()) -> tree().
  
positive.code: %%% Types
  %%%===================================================================
  
  -record(contract_tree, {}).
  
  -opaque tree() :: #contract_tree{}.
  
  
negative.code:     Tree.
  
  -spec root_hash(tree()) -> {ok, aeu_mtrees:root_hash()} | {error, empty}.
  root_hash(#contract_tree{contracts = CtTree}) ->
      %% The contract tree references the hashes of the call trees so we only
      %% need the hash of the contract tree here.
      aeu_mtrees:root_hash(CtTree).
  
  