Updating fit with lmer()

Suppose I run a linear mixed-effects model such as

(fm <- lmer(Reaction ~ Days + (Days|Subject), sleepstudy))

and I want to update the fitting with a new response data (e.g.,
newReaction) with everything else the same as the original model. Dr. Bates
suggested the following on Sept. 5, 2007 to save runtime:

After fitting the model to the first simulated response, producing the

object 'fm', ?the only operations needed to update the model are

fm at y <- newy
Xy <- cbind(fm at X, fm at y)
fm at ZtXy <- fm at Zt %*% Xy
fm at XytXy <- crossprod(Xy)
lme4:::mer_finalize(fm, verbose)

However, I could not find slots fm at ZtXy and fm at XytXy any more in lme4 nor
lme4a. Have these two slots been changed to different names?

Or essentially the above steps have been wrapped into refit() in lme4?