function b = bootstrap(x,stat,nboot);
% usage b = bootstrap(x,stat,nboot)
%     x = original data matrix
%     stat = stat function ('mean')
%     nboot = number of bootstrap resamples
n = size(x,1);
for i=1:nboot
   xs = x(resample(n),:);
   b(i,:) = feval(stat,xs);
end