/*
 *  call-seq:
 *     dvector.last     ->  number or nil
 *     dvector.last(count)  ->  a_dvector
 *  
 *  Returns the last element, or the last _count_ elements, of _dvector_. If the vector is empty,
 *  the first form returns <code>nil</code>, and the second returns an empty vector.
 *     
 *     a = Dvector[ 1, 2, 3, 4, 5 ]
 *     a.last   -> 5
 *     a.last(1)   -> Dvector[ 5 ]
 *     a.last(3)   -> Dvector[ 3, 4, 5 ]
*/ VALUE dvector_last(int argc, VALUE *argv, VALUE ary) {