What are unfoldable Tboxes?

To give credit where credit is due, know that I simply googled for "Unfoldable TBox" to find an example on the Web. I found a slide set by Boontawee Suntisrivaraporn, entitled "Introduction to the Semantic Web - Description Logics" where you find on slides 15 and 16 the following definition and examples (I improve the examples a bit):

Unfoldable TBox: Only concept denitions can be used, and the following conditions must be respected: (denitional) there can be at most one denition for each concept name, for instance, it is impossible to have A ≡ C and A ⊑ D in the TBox; (acyclic) there is no cyclic dependency in the TBox, for instance, it is impossible to have A ≡ B ⊓ C and B ≡ ∃r.A in the TBox.

This is an example of an unfoldable TBox:

Man ≡ Person ⊓ Male
Male ≡ ∀hasGender.{♂}
Woman ≡ Person ⊓ Female
Female ≡ ∀hasGender.{♀}
Parent ≡ Person ⊓ ∃child.Person

Remark that there are things on the left hand side that appear on the right hand side. If a concept appears on the right hand side and is also defined on the left hand side, it can be replaced by its definition, e.g.:

Man ≡ Person ⊓ ∀hasGender.{♂}

and it can be done recursively, which is what is called "unfolding". However, the following is not unfoldable:

Man ≡ Person ⊓ Male
Woman ≡ Person ⊓ Female
Parent ≡ Person ⊓ ∃child.Person
Person ≡ Man ⊔ Woman

because from the fourth axiom defining Person, one would replace Man and Woman by concepts that contain Person, which would then unfold to Man ⊔ Woman again. It's cyclic, thus not unfoldable.