Introduce optFlag so that one can choose not to build the optimization-related cell of matrices called B. It is too computationally costly and should probably be done in a different way.

function A = spdiag(a,i)
    default_arg('i',0);

    if isrow(a)
        a = a';
    end

    n = length(a)+abs(i);
    A = spdiags(a,i,n,n);
end