⊦ fd ∈ abs-of(of) ↔ fd ∈ of
⊦ abs-of(∅)[fd] = (∅)[fd]
Used simplifier rules: abs-of-in: ⊦ fd ∈ abs-of(of) ↔ fd ∈ of;% ⊦ ¬ fd ∈ ∅; (s)% ⊦ ¬ fd ∈ ∅; (s)