generic specification
   parameter elem
   using nat
target
   sorts Set;
   constants 
       ∅ : Set;
   
   functions 
       . ++ . : Set × Elem → Set prio 9 left;
       χ : Set → Elem → Bool;
       { . } : (Elem → Bool) → Set;
   
   predicates 
       . ∈ . : Elem × Set;
       . ⊆ . : Set × Set;
   
   variables 
       s : Set;
       a : Elem;
       b : Elem;
       f : Elem → Bool;
   
   
   induction 
      Set generated by { };
   
   axioms 
      characteristic-function: χ(s)(a) ↔ a ∈ s;
      used for: s, ls;
      
      construct-set: a ∈ {f}  ↔ f(a);
      used for: s, ls;
      
      Extension: s1 = s2 ↔ (∀ a. a ∈ s1 ↔ a ∈ s2);
      used for: ls;
      
      In-empty: ¬ a ∈ ∅;
      used for: s, ls;
      
      In-insert: a ∈ s ++ b ↔ a = b ∨ a ∈ s;
      used for: s, ls;
      
      Subset: s1 ⊆ s2 ↔ (∀ a. a ∈ s1 → a ∈ s2);
      used for: ls;
   
   
end generic specification