function v = hhBuild(x,position);
% function v = hhBuild(x,position);
% compute v representing a householder to zero out entries k+1:end
% putting all the length into position k.

n = length(x);
k = position;
if x(k)<0; s=1; else; s=-1; end 
target = [x(1:k-1) ; norm(x(k:end))*s ; zeros(n-k,1)];

v = x - target;

