I came across this excerpt from Pliny, Epistulae 5.16 in Wheelock's Latin. It is the last sentence that is puzzling me. For awhile, I was stumped by tam iusto but I think I understand it now. It is the ablative modifying dolore, something like "If you write to him about his so justified grief…" I still do not understand the role of quas. Is it the relative pronoun or is it the indefinite pronoun quis, quid used after si? The latter interpretation does not make sense to me because I would have expected an indefinite adjective not a pronoun "some letter." Could someone explain to me the function of quas here? I would be most appreciative.