Optional Readonlycontainsthe argument carrying the direct superclass(es): S4 contains, S7 parent, R6 inherit, RC contains
Optional Readonlymemberthe arguments carrying members, in the order they are to be read
Optional Readonlynamethe argument naming the class
Optional Readonlyprototypethe argument carrying the member defaults (S4 prototype)
Optional Readonlyrelationthe declaration relates existing classes rather than declaring one
ReadonlysystemOptional Readonlyunionthe argument listing the members of a class union (setClassUnion)
Optional Readonlyvirtualthe argument saying the class cannot be instantiated (S7 abstract)
How one built-in declares a class, so no processor has to guess an argument's meaning from its position.