After initial compilation, a cluster in the Ace file is changed from
the form "all root_cluster" to a list of the two specific clusters in
the root cluster directory.  Recompilation works fine.  However, when
the Ace is then changed to indicate a new root class and and the class
name inside one file is also changed, a second recompile ends with a
segmentation fault in `backup_directory'

Discovered in Release 5.0.015.