+ * Arguments: @mp *m@ = a suggested destination integer
+ * @size_t sz@ = size required for result, in digits
+ * @unsigned f@ = various flags
+ *
+ * Returns: A pointer to an appropriate destination.
+ *
+ * Use: Converts a suggested destination into a real destination with
+ * the required properties. If the real destination is @d@,
+ * then the following properties will hold:
+ *
+ * * @d@ will have exactly one reference.