Problem 44345. MATLAB Counter

My function works directly as described - but does not manage to pass the test suite ... what is wrong?

%%
function varargout = counter(varargin)
persistent add base
if isempty(varargin)
base = base + add;
varargout{1} = base;
end
if numel(varargin) > 1
add = varargin{2};
base = varargin{1}-add;
end
end

For anyone needing a gentle refresher (like I did) on nested anonymous functions:
https://www.mathworks.com/matlabcentral/cody/problems/24-function-iterator

Cody's Problem 24 might be of relevance, but note that so far very few of the correct solutions to the present problem (Problem 44345) have used anonymous functions, whereas all of the correct solutions have used named functions (some nested, some not).

Correction: there is a solution here (Solution 1308690) which only uses an anonymous function, without calling a named user function.

Either (or both) of the following pages may be of interest in tackling this problem: https://mathworks.com/help/matlab/matlab_prog/nested-functions.html and https://mathworks.com/help/matlab/matlab_prog/share-data-between-workspaces.html .

It's the best problem in Cody:Easy