Right now, Abs just sets parents :: (), but parameterizing it gives us some more flexibility for systems that have multiple roots.
E.g.,
Abs () -- unix paths
Abs Drive -- Windows paths
Abs (These Project Branch) -- Unison paths
Abs Host -- URL paths
And there can be conversions, like Abs () -> Abs Drive, which converts a drive-independent “absolute” path on Windows into a fully-qualified path with drive letter.