I am trying to understand this line, which is from Questa o Quella, an aria from Verdi's Rigoletto. If you are unaware, this is quite an old text.
sol chi vuole si serbe fidele
I am struggling specifically with the role that the word 'si' plays here.
I have a couple of ideas, but I am afraid my level of Italian is quite low.
I am not sure if it means 'only those who wish should remain faithful to each other', 'only they who themselves wish to should remain faithful', or whether there is an archaic meaning of si that I am unaware of.
So could somebody please explain the role of si in this sentence.
Thank you in advance