TreeAutomaton#toString changes state

[akoehn]

I had some stuff to debug and noted that the program behaved differently under the debugger than without. toString iterates over rules by calling getRuleSet, which in turn calls makeAllRulesExplicit.

In my case, this lead to this warning:

** WARNING ** Invalid use of TreeAutomaton#makeAllRulesExplicit.

It would be great to have this fixed, but I don't know what changing TreeAutomaton#toString affects. As is, this has already cost the German taxpayer quite some money.