Requirements on range concept

Class R implementing the concept of range must define:

• R::R( const R& ); 

  Copy constructor

• R::~R(); 

  Destructor

• bool R::is_divisible() const; 

  True if range can be partitioned into two subranges

• bool R::empty() const; 

  True if range is empty

• R::R( R& r, split ); 

  Split range r into two subranges.