/* 
 *  call-seq:
 *     dtable[row,col]  ->  number or nil
 *     dtable.at(row,col)  ->  number or nil
 *
 *  Returns the element at location _row_, _col_.  Returns +nil+
 *  if the location is out of range.
 */ VALUE dtable_at(VALUE ary, VALUE xloc, VALUE yloc) {