actualize set-union with gref by morphism : set → refset; : elem → ref; ∅ → ∅; ++ → ++; # → #; { } → { }; ∪ → ∪; ∩ → ∩; \ → \; -- → --; ∈ → ∈; ⊆ → ⊆; s → rs; s0 → rs0; s1 → rs1; s2 → rs2; a → r; b → r0; c → r1; end actualize