checking for bound variables in the set expression; handling negation more generally

set_comprehension_pointfree simproc now handles the complicated test case; tuned

refined conversion to only react on proper set comprehensions; tuned

moving Pair_inject from legacy and duplicate section to general section, as Pair_inject was considered a duplicate in e8400e31528a by mistake (cf. communication on dev mailing list)

employing a preprocessing conversion that rewrites {(x1, ..., xn). P x1 ... xn} to {(x1, ..., xn) | x1 ... xn. P x1 ... xn} in set_comprehension_pointfree simproc

another Future.shutdown after Future.cancel_groups (cf. 0d4106850eb2);