inline convert_ts2ba_label
use set comprehension
extract find_ba_succ, find_prod_succ

to modularize, so that the successor-generating functions (that implement the semantics of BA * TS) can be shared among the explicit and on-the-fly products
replace MathSet by set, because mutable states not supported any more
more logging in transys.products
bugfix: FSA.atomic_propositions should point to alphabet.math_set