iate normal distributions. The
distributions differ only by their x- and y-displacement, that is, by
their mean values. There are two predictors <code>grp1</code> and
<code>grp2</code> which predict the differences in means.
<code>grp1</code> predicts differences in the first dimension and
<code>grp2</code> predicts differences in the second dimension. Without
focus parameter, both predictors are needed to distinguish all four
groups. If one of the two means is chosen as a focus parameter, only one
of the t