Unused `_root` argument in DAG cost extractors?

[ren-zeng]

Hi I'm a egglog user interested in extracting a list of terms from a saturated e-graph under DAG cost (account for sharing). It seems that egglog does not natively support DAG cost but fortunately extraction-gym does contain some implementations.

I see several implementation of Extractor such as GlobalGreedyDagExtractor ignores the argument _root (which I guess is the e-class IDs corresponding to the list of terms to extract).

Here are my questions:

1. Do these implementations extracts all the e-classes in the e-graph under DAG cost?
2. If I only want to select a subset of e-classes as the extracted terms, should I first prune the e-graph myself and call extract on the pruned e-graph?