Some things not stated (or not clear) in Clicks Diss:

Some things not stated (or not clear) in Clicks Diss:
 - when splitting partitions, n-input Identities might lose
   its follower state -> place them on cprop
 - the race split might choose the split out the part NOT containing the
   split list g. This destroys split_by(), so we have to exchange X and X' if
   this happens
 - in cause_split() we have to removed all followers from the touched list:
   split() doesn't work with them AND we must check the number of elements an split
   with the number of leaders (last part is clearly stated in the Diss)
 - a node can only be a follower if the leader is in the same partition!
 - !constant for leader -> follower transition is misleading: must be neither
   Top nor const (no followers in Top and const partitions)

Other changes:
 - we don't have to put on the end of the unwalked set in step(), just NOT
   at the start
 - combined add_to_touched() and add_to_partition_touched()
 - we handle n-input identities in segregate_def_use_chain(), no need for
   extra handling in segregate_def_use_chain_1()
 - BugFix: ensure, that a node after follower -> leader transition is only
   placed on fallen once
 - BugFix: errouniously deleted add_to_worklist(Y) if old type was T or Const
 - clean keep-alives

[r21061]