trait Internals { this: Module =>
  val io = IO(new Bundle{ val a = Input(Bool()) })
}
class Foo extends Module with Internals with FlattenInstance
class Bar extends Module with Internals {
  val baz = Module(new Baz)
  baz.io.a := io.a
}
class Baz extends Module with Internals
/* The resulting instances will be:
     - Top
     - Top.x
     - Top.y
     - Top.z
     - Top.z.baz */
class Top extends Module with Internals {
  val x = Module(new Foo)                      // x will be flattened
  val y = Module(new Bar with FlattenInstance) // y will also be flattened
  val z = Module(new Bar)                      // z will not be flattened
  Seq(x, y, z).map(_.io.a := io.a)
}