anchor.diff:                      fun( N, Delta0 ) ->
                         acc_delta( N, Delta0, Ret, Prefix )
                       end,
  -                    #{}, maps:keys( Ret ) )
  -
  -
  +                    #{}, maps:keys( Ret ) ),
  +          reduce( X1, {Rho, Mu, Gamma, maps:merge( Omega, Delta )} )
         end
     end.
   
  
positive.code: %% Internal Functions
  %% =============================================================================
  
  
  -spec reduce( X0, Rho, Gamma ) -> [cf_sem:str()]
  when X0    :: [cf_sem:expr()],
  
negative.code:      Gamma :: #{string() => cf_sem:lam()}.
  
  reduce( X0, Rho, Gamma ) ->
    reduce( X0, {Rho, fun cre:submit/1, Gamma, #{}} ).
  
  -spec reduce( X0, Theta ) -> [cf_sem:str()]
  when X0    :: [cf_sem:expr()],
       Theta :: cf_sem:ctx().
  
  reduce( X0, {Rho, Mu, Gamma, Omega} ) ->
    X1 = cf_sem:eval( X0, {Rho, fun cre:submit/1, Gamma, #{}} ),
    case cf_sem:pfinal( X1 ) of
      true  -> X1;
      false ->
        receive
          {failed, ActScript, Out} -> error( {failed, ActScript, Out} );
          {finished, Summary} ->
            Ret = maps:get( ret, Summary ),
            Prefix = maps:get( prefix, Summary ),
  