Let’s consider the main example of the Dominator article of wikipedia:

And let’s consider the following definitions taken from the same article:

• A node d strictly dominates a node n if d dominates n and d does not equal n.

• The dominance frontier of a node d is the set of all nodes ni such that d dominates an immediate predecessor of ni, but d does not
  strictly dominate ni. It is the set of nodes where d’s dominance
  stops.

What is the dominance frontier of node 1?

My understanding is that it is undefined simply because node 1 strictly dominates 2, 3, 4, 5 and 6 – which remove them all from consideration because of the last part of the dominance frontier definition “and d does not strictly dominate ni”.

This also applies to the dominance frontier of node 2.

Is it expected that there is no dominance frontier for these nodes?