actualize set-union
with references
by morphism : elem → references; : set → referencesset; ∅ → ∅; ++ → ++; # → #; { } → { }; ∪ → ∪; ∩ → ∩; \ → \; -- → --; ∈ → ∈; ⊆ → ⊆; a → refs;
b → refs1; c → refs2; s → refsset; s0 → refsset0; s1 → refsset1; s2 → refsset2;
end actualize