Existing practice

Can divide existing tools into three categories:
History browsers - look at the time evolution of one file
Cross-referencers - make connections between files, but only at one time
Merge tools - assist in combining two branches of one file

Cross referencers and merge tools are more advanced
These tasks are difficult without sophisticated assistance
A poor merge tool can be worse than none at all
A poor history browser can be worked around

So we're going to concentrate on improving history browsers