/*
 *  call-seq:
 *     dvector.first          ->  number or nil
 *     dvector.first(count)   ->  a_dvector
 *  
 *  Returns the first element, or the first _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.first   -> 1
 *     a.first(1)   -> Dvector[ 1 ]
 *     a.first(3)   -> Dvector[ 1, 2, 3 ]
 */ 
VALUE dvector_first(int argc, VALUE *argv, VALUE ary) {