[gclist] GC triggering strategies

[boehm.PARC@xerox.com]

The other things to keep in mind are:

1. Triggering GC based on stack height etc. makes less sense in a multithreaded
system  (which stack?)

2. Providing the user with explicit calls is tricky if you want the interface
to be independent of the collector.  For example, an incremental update
collector doesn't trace any particular program state.

That's not to say that there aren't situations in which these techniques are
very useful.

Hans