Forest of "prune trees" where a prune tree is a location that can be pruned with a tree of descendants that must be excluded from the pruning.

Internally we store this as a single tree of booleans with the following characteristics: * 'true' indicates a location that can be pruned, possibly with some excluded descendants. * 'false' indicates a location that we should keep (i.e. exclude from pruning). * 'true' (prune) cannot be a descendant of 'false' (keep). This will trigger an exception. * 'true' cannot be a descendant of 'true' (we'll just keep the more shallow 'true'). * 'false' cannot be a descendant of 'false' (we'll just keep the more shallow 'false').