enrich
   map, 
   set-union
with
   functions 
       . \ . : Map × Set → Map;
       . | . : Map × Set → Map;
       dom . : Map → Set;
       . ⊕ . : Map × Map → Map;
   
   predicates 
       . ∈ . : Data × Map;
   
   variables 
       a : Elem;
       b : Elem;
       c : Elem;
       st : Map;
       d : Data;
       s : Set;
       n : Nat;
   
   axioms 
      Dom: a ∈ dom st ↔ a ∈ st;
      used for: s, ls;
      
      Diff-base: st \ ∅ = st;
      used for: s, ls;
      
      Diff-rec: st \ (s ++ a) = (st \ s) -- a;
      used for: ls;
      
      Data-in: d ∈ st ↔ (∃ a. a ∈ st ∧ st(a) = d);
      used for: ls;
      
      restrict: st | s = st \ (dom st \ s);
      used for: ls;
      
      oplus-l-in: a ∈ st0 ⊕ st1 ↔ a ∈ st0 ∨ a ∈ st1;
      used for: s, ls;
      
      oplus-l-at-left: a ∈ st0 → (st0 ⊕ st1)(a) = st0(a);
      used for: s, ls;
      
      oplus-l-at-right: ¬ a ∈ st0 ∧ a ∈ st1 → (st0 ⊕ st1)(a) = st1(a);
      used for: s, ls;
   
   
end enrich