function qb=SparseQMult(c,s,r,p,b)
% Return the product Qb where Q is defined
% as a collection of Givens rotation
% [c(i) s(i);-s(i) c(i)] to zero the (r(i),p(i))
% entry.
qb=b;
for i=length(c):-1:1
qb([p(i);r(i)])=[c(i) -s(i);s(i) c(i)]*qb([p(i);r(i)]);
end