/*
 *  call-seq:
 *     dtable.add(number)       ->  a_dtable
 *     dtable.add(other)        ->  a_dtable
 *     dtable + number          ->  a_dtable
 *     number + dtable          ->  a_dtable
 *     dtable + other           ->  a_dtable
 *  
 *  When argument is a number, this operation returns a copy of _dtable_ with each entry x replaced by x + _number_.
 *  When argument is a data array, this operation returns a copy of _dtable_ with each entry x replaced
 *  by x + the corresponding entry in the _other_ data array.
 */ VALUE dtable_add(VALUE ary, VALUE arg) {