@logl logl
@byobs
res=xy-c(1)
sig2=c(2)+c(3)*res(-1)^2+c(4)*sig2(-1)
z=res^2/sig2/(k(1)-2)+1
logl=@gammalog((k(1)+1)/2)-@gammalog(k(1)/2)-log(3.14159265359)/2-log(k(1)-2)/2-log(sig2)/2-(k(1)+1)*log(z)/2