/*
 *  call-seq:
 *     dvector.neg   ->  a_dvector
 *  
 *  Returns of copy of _dvector_ with each entry x replaced by -x.
 *     
 *     a = Dvector[ 1, 2, 3, 4 ]
 *     a.neg   -> Dvector[ -1, -2, -3, -4 ]
 */ 
VALUE dvector_neg(VALUE ary) {