function b = mcbdraw(y,x,sig,sdraw)
% function b = mcbdraw(y,x,sig,sdraw)
% produces a draw from the normal posterior of b conditional on everything else.
ns=size(sig,2);
T=size(y,1);
nx=size(x,2);
ywt=y./sig(sdraw)';
xwt=x./repmat(sig(sdraw)',1,nx);
bhat=xwt\ywt;
[u,d,v]=svd(xwt);
b=bhat+(v/d(1:nx,1:nx))*randn(nx,1);
