Class Body implementing the concept of parallel_scan body must define:
- Body::Body( Body&, split );  
- bso that- thisand- bcan accumulate separately
-  Destructor
- void Body::operator()( const Range& r, pre_scan_tag );  
- r
- void Body::operator()( const Range& r, final_scan_tag );  
- r
- void Body::reverse_join( Body& a );  
- ainto- this, where- awas created earlier from- bby b's splitting constructor