protected function computeParameters($xs)
 {
     if (!is_array($xs[0])) {
         $xs = $this->arrayNonArray($xs);
     }
     $xs_columns = ll_transpose($xs);
     foreach ($xs_columns as $index => $column) {
         $this->mean[$index] = ll_mean($column);
         $this->variance[$index] = ll_variance($column);
     }
 }
function __ll_flip($rows)
{
    return ll_transpose($rows);
}