greatest bisimulation (K minimization)

Question

The Confusion 2022 september 6th protokol

Protokol Question: 

Minimize the following Kripke structure K3 over variables {a, b, d} by the

quotient construction of the greatest bisimulation relation between K3 and

itself.

Hint: If your relation is closed under reflexivity or symmetry, you may add a

note that all reflexive entries and/or symmetric duplicates shall be considered.

My doubt: When I check (S2, S3): So why does (S2, S3) fail the Step 1 refinement check?

Answer 1

S2 has a transition to itself which cannot be simulated by S3 since S3 has only a transition to S4 which is not in relation with S2. For this reason, the pair (S2,S3) is removed in step 1.