Body implementing the concept of parallel_scan body must define: -Body::Body( Body&, split );
b so that this and b can accumulate separately -Body::~Body();
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 );
a into this, where a was created earlier from b by b's splitting constructor