- finally understand HOW n-input nodes made the Follower -> Leader transition

- finally understand HOW n-input nodes made the Follower -> Leader transition
  (read the text until end :-), this removes half of the fixes from r21061;
  this simplifies segregate_def_use_chains()
- add a partition checker
- opcode contains the arity now (not clear if this is really needed)
- a lot of more assertions()
- do not kill Confirms yet

Still buggy yet :-(

[r21069]