Original sentence: As Megill mentions on his website, Metamath's axioms and theorems are actually schemes; their variables are generic variables, and any appropriate substitution can be made for them..
Edited sentence: As Megill mentions on his website, Metamath's axioms and theorems are actually schemes; their variables are generic variables, and any appropriate substitution can be made for them.. Some proof softwares don't use schemes but it doesn't seem very natural since using schemes is the normal way to do mathematics for a human being