getParents and getDependencies do not have an `omit` argument

[paciorek]

This is inconsistent with nimble.

Handling this might involve having intersect/setdiff functionality for varRanges (seemingly -- not nodeRanges as graph traversal involves graph rules that work at the level of parts of variables).

[paciorek]

We do have exclude, which can work with varRanges. And fracture which takes a rule and a range. Both return rules, but perhaps the core logic can be used for working with ranges.