), opU) ;
      y = linsolve (U, linsolve (L, c(p,:), opL), opU) ; toc

%% INV is easy to use, but slow and inaccurate
%
% Oh bother!  Using LU and LINSOLVE is too complicated.  You just want to
% solve your system.  Let's just compute inv(A) and use it twice.  Easy to
% write, but slower and less accurate ...

S = inv (A) ;
x = S*b ; norm (b-A*x)
y = S*c ; norm (c-A*y)

%%
% Sometimes using the inverse seems inevitable.  For example, your textbook
% might show the Schur complement formula as S = A-B*inv(D)