Merge remote-tracking branch 'origin/eddie/abc9_refactor' into eddie/abc9_required

diff --cc backends/aiger/xaiger.cc
index 66ddbde33442b99a148237760d26aaa417e11163,a9b75ecc7b14eee8a16fb4eb16a022b9250f09c7..71b6524124b7f80075be76fc4534c2c29517fc4f
--- 1/backends/aiger/xaiger.cc
--- 2/backends/aiger/xaiger.cc
+++ b/backends/aiger/xaiger.cc
@@@ -234,7 -234,7 +234,8 @@@ struct XAigerWrite
                                                        box_list.resize(abc9_box_seq+1);
                                                box_list[abc9_box_seq] = cell;
                                                // Only flop boxes may have arrival times
 +                                              //   (all others are combinatorial)
+                                               abc9_flop = inst_module->get_bool_attribute("\\abc9_flop");
                                                if (!abc9_flop)
                                                        continue;
                                        }