Help with unfolding / selecting map's values

[tomiasp]

I have a pretty basic query similar to this:

g.V().hasLabel('Person')
    .group().by('LastName')

This generates a map of keys and all the occurrences of Person vertices in they key's value as array that share the same last name.

In normal TinkerPop, I can do this:

g.V().hasLabel('Person')
    .group().by('LastName')
    .unfold() // Make last name its own array, so we can process them later individually as groups
    .select(values) // Get rid of the key, flatten the object
    .map(
        coalesce(
            unfold().has('PropertyX', 'valueWeWantToPrefer'), // Prefer to choose based on this
            unfold().has('PropertyX', 'default')              // Else choose this vertex
        )
    )

How can I achieve the unfold().select(values) behaviour with Gremlinq? Gremlinq does not allow Unfold() operation on Maps. even though it is possible in TinkerPop https://tinkerpop.apache.org/docs/3.7.1/reference/#unfold-step

Also, I can't figure out how to implement the select(values) step...