Is semen discharged after pre-seminal fluid impure? And is semen, and the garment, ruled to be impure in the case of a wet dream if the discharge of pre-seminal fluid is not known?

"Madhi (pre-seminal fluid) is impure (najis), while maniyy (seminal fluid) is pure according to the preponderant opinion. The passage of maniyy from the exit point of madhi does not render it impure. And whoever has a wet dream but sees no trace (of seminal fluid) is not obligated to do anything. If he does see maniyy, it is pure, and the mere doubt of madhi having been discharged does not make it impure, because the original state is purity."