+ if (SCM_NULL_OR_NIL_P (lst))
+ return new_tail;
+
+ /* SCM_VALIDATE_LIST would run through the whole list to make sure it
+ is not eventually circular. In contrast to most list operations,
+ reverse! cannot get stuck in an infinite loop but arrives back at
+ the start when given an eventually or fully circular list. Because
+ of that, we can save the cost of an upfront proper list check at
+ the price of having to do a double reversal in the error case.
+ */
+
+ while (scm_is_pair (lst))