Clarify auxiliary variable types

Related to Dynare/dynare#1811

(cherry picked from commit 58ee2400)

matlab/isauxiliary.m

@@ -20,13 +20,13 @@ function b = isauxiliary(var, types)
 %   -  4     Expectation (substitute for Expectation Operator)
 %   -  5,    Diff forward (substitute for the differentiate of a forward variable)
 %   -  6,    Multipliers for FOC of Ramsey Problem
-%   -  7,    Variable for var_model with order > abs(min_lag()) present in model
+%   -  7,    currently not used, see SymbolTable.hh
 %   -  8,    Variable for Diff operator
 %   -  9,    Lag on Diff
-%   - 10,    Unary operator (log, exp)
+%   - 10,    Variable created when diff was taken of unary operator (log, exp)
 %   - 11,    Lead on Diff
 
-% Copyright (C) 2018-2019 Dynare Team
+% Copyright (C) 2018-2021 Dynare Team
 %
 % This file is part of Dynare.
 %

matlab/subst_auxvar.m

@@ -60,6 +60,9 @@ if ~isempty(aux_index)
                 str = sprintf('%s', M_.endo_names{M_.aux_vars(aux_index).endo_index});
             end
             return
+        case 7
+            % currently unused
+            error('This type of auxiliary variable should not occur, please contact the developers.')
         case 8
             % Diff operator
             str = sprintf('diff(%s)', M_.endo_names{M_.aux_vars(aux_index).orig_index});
@@ -74,6 +77,9 @@ if ~isempty(aux_index)
             end
             str = sprintf('diff(%s(-%u))', M_.endo_names{M_.aux_vars(j).orig_index}, lags);
             return
+        case 10
+            % Variable created when diff was taken of unary operator (log, exp)
+            error('This type of auxiliary variable should not occur, please contact the developers.')
         case 11
             % Leaded diff
             leads = 0;