[jhc] Occurance collection.

[Lemmih]

Hiya,

In E.SSimplify.collectOccurance the 'arg' function is used quite often
and I can't tell why. The function clears usage information for free
variables and is primarily used for types.
It would be a great help if you'd add a short description to
collectOccurance and collectDs. Any information about invariants
and/or what they're trying to do would be appreciated.

-- 
Cheers,
 Lemmih