/*
 *  call-seq:
 *     dtable.sub!(number)       ->  dtable
 *     dtable.sub!(other)        ->  dtable
 *  
 *  When argument is a number, each entry x in _dtable_ is replaced by x - _number_.
 *  When argument is a data array, each entry x in _dtable_ is replaced by x - 
 *  the corresponding entry in the _other_ data array.
 */ VALUE dtable_sub_bang(VALUE ary, VALUE arg) {