Optional ReadonlyidxOptional Readonlymethodsevery entry of this argument is a method, whatever it looks like (RC's methods)
Optional ReadonlynameOptional Readonlytypedthe entries state a type (S4 slots, RC fields), rather than a value or a function (R6, RC methods)
Optional Readonlyvisibilitythe visibility every entry of this argument has (R6's public/private/active)
A member-carrying argument, plus how its entries are to be read.