title: on the predict.merMod function arguments
body: <p>In the function <code>predict.merMod</code> of the <code>lme4</code> package, what is the difference between  the following arguments: <code>allow.new.levels=TRUE</code>, <code>re.form=NA</code> and <code>re.form=~0</code> if we have only a random intercept?</p>