  Limits of graphs were initiated recently in the two extreme contexts of dense
and bounded degree graphs. This led to elegant limiting structures called
graphons and graphings. These approach have been unified and generalized by
authors in a more general setting using a combination of analytic tools and
model theory to FO-limits (and X-limits) and to the notion of modeling. The
existence of modeling limits was established for sequences in a bounded degree
class and, in addition, to the case of classes of trees with bounded height and
of graphs with bounded tree depth. These seemingly very special classes is in
fact a key step in the development of limits for more general situations. The
natural obstacle for the existence of modeling limit for a monotone class of
graphs is the nowhere dense property and it has been conjectured that this is a
sufficient condition. Extending earlier results we derive several general
results which present a realistic approach to this conjecture. As an example we
then prove that the class of all finite trees admits modeling limits.
