FSM

Are there any redundant states? Can this FSM be simplified?

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All the state have implicit loopback which means e.g. that if we are in S0 and a='0' and b='0' then we will stay in S0.

Answer, Yes :-)

State S4 is redundant.

Note State S3 is not a redundant state but a state that can be merged into S1, which synthesis tool can do this?