/* 
 *  call-seq:
 *     dvector.at(int)  ->  number or nil
 *
 *  Returns the element at index _int_. A
 *  negative index counts from the end of _dvector_.  Returns +nil+
 *  if the index is out of range.
 *
 *     a = Dvector[ 1, 2, 3, 4, 5 ]
 *     a.at(0)     -> 1
 *     a.at(-1)    -> 5
 */ VALUE dvector_at(VALUE ary, VALUE pos) {