{2}, ..., Y{NY}|. Every |Y{i}| is assumed to be 
% symmetric and thus only the *lower triangle is vectorized column-wise*. 
% If any callback functions described below asks for derivatives with 
% respect to |k| (|k>Nx|), it means it's one of the variables in one of the 
% matrices |Y{i}|.
%
% *Optional problem data*
%
% * [optional] *problem name and a comment* (e.g. source) just for logging
%   purposes. If probname is empty or not present, current time&date and
%   default text is used, comment