> I always tend to use patterns and not think of expressions).

well actually you were using an expression already, you had

group-adjacent="@class"

which means group all the _para_ elements with the same class. note that
the nodes being grouped don'e match @class, they just evalate @class to
teh same value.

On the other hand, if you wanted to group thiongs such that every group
of nodes with a class of title started a new group, you wouldn't say

group-starting-with="@class='title'" (a boolean valued expression) you
say

group-starting-with="para[@class='title']" a pattern that matches  (or
dees not match) the nodes being grouped.

David