enrich
   store, 
   set-union
with
   functions 
       . \ . : store × set → store;
       . | . : store × set → store;
       dom . : store → set;
       . ⊕ . : store × store → store;
   
   predicates 
       . ∈ . : data × store;
   
   variables 
       a : elem;
       st : store;
       d : data;
   
   
   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