What is the derivation of 뭔? It looks like it might be a short form of 무엇는, in the same way that 뭘 is short for 무엇을, but I see it used in this sentence by a native speaker: 뭔가 제가 모르는 일이 있었어요? which has the subject marker "가" after 뭔 which would be redundant, right? Thanks, John.