-# $sort_key is either a column name, or integer 1 which simply
-# sorts the output according to the leftmost column not containing
-# any undef. if not specified, we do not change order.
+# $sort_key can be used to sort after a column, if it isn't set we sort
+# after the leftmost column (with no undef value in $data) this can be
+# turned off by passing 0 as $sort_key