Limit on the number of times to self-compose a set
of signals that form a strongly connnected component in the dependency graph,
i.e. they all depend on each other. By default this is NIL, which means that
we will compose together such an SCC N times where N is the size of the SCC --
this is the theoretical limit on how many iterations are needed to come to a
fixed point (assuming the expressions are X-monotonic). But since this
behavior can cause a blowup in the size of the expressions, setting the limit
to a small number is usually practical if there is such a problem.