get rid of tryGen

[jvoigtlaender]

Various places in the code use tryGen, erroring out in case a certain number of attempts does not lead to a random value with a desired property. Hopefully, such a runtime error will never be encountered in real, because it would crash "the whole thing". So, it would be good to:

• analyze whether indeed all these uses of tryGen are with predicates known to be satisfiable at their use sites,
• then probably replace tryGen with suchThat.