function K = hmmphicoef(A,B,C,n,Xt)
P = hmmfillprob(A,B,C,n);
F = P*0;
m = size(B,1);
[Ef1,K1] = condexp(P,F,Xt         ,m,n);
[Ef0,K0] = condexp(P,F,Xt(1:end-1),m,n);
K = K1-K0;
return