%% generate a random sparse triang. matrix
 n = 1000;
 A = sprand(n,n,5/n);
 L = tril(A,-1);
%%-------------------- .. and a sparse RHS
 b = sprand(n,1,5/n);
%%-------------------- get list 
 [list, M] = TopSort(L,b);
 list
 %%-------------------- solve (I+L)x = b
 x = spLsolv(L,b,list);
%%-------------------- compare with exact sol.
 x0 = (eye(n)+L)\b;
 norm(x-x0)
