'slot'. In the top file, you can how give' the virtual node's type as
type:count - sim:5, for example. If you give no count, it defaults to
one.

The idea here is to allow a pre-pass to assign to 'coarsen' the
virtual graph. This pre-pass could, for example, combine several
virtual or simulated nodes into one so that assign has a simpler
virtual graph to work with.