simp only [Array.size_mkArray]
n : Nat
arr : Array (Option (DefaultClause n))
hsize : (ofArray arr).assignments.size = n
modified_assignments_invariant : Array Assignment → Prop :=
  fun assignments =>
    ∃ hsize, ∀ (i : PosFin n) (b : Bool), hasAssignment b assignments[i.val] = true → unit (i, b) ∈ (ofArray arr).toList
⊢ (mkArray n unassigned).size = n
no goals