Sentence:  For example, for Fixnum, succ returns the current number plus one. This method is necessary if you want to make your class to work with ranges. To have a “<WSD>rangeable</WSD> class,” you must also provide an implementation to the <=> operator (which is what the range uses internally to know when it should stop). 
Meaning of the word marked with <WSD> tags:  Supporting the use of ranges.