rix constraint doesn't depend on the variable
%   and also to carefully distinquish zero matrix (|sparse(m,m)|) and empty
%   matrix |[]|.
% * a 2nd derivative of the |k|-th nonlinear matrix constraint (|k=1..NANLN|)
%   w.r.t. |i,j|-th variables (|i,j=1..Nx+NYnnz|) as a dense or sparse
%   symmetric matrix of the appropriate dimension or empty matrix |[]|.
%   |mconhess()| will be called only with the indices |i,j| where the first
%   matrix derivative was nonempty. Alternatively, |mconlagrhess()| c