This method has no unique definition. Generally speaking, methods which
always return the same type usually don't use #class, but #species.
For example, a PositionableStream's species is the class of the collection
on which it is streaming (used by upTo:, upToAll:, upToEnd). Stream uses
species for obtaining the class of next:'s return value, Collection uses
it in its #copyEmpty: message, which in turn is used by all collection-re-
turning methods. An Interval's species is Array (used by collect:, select:,
reject:, etc.).