+ * 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.
+ *
+ * * If @m@ is not @MP_NEW@, then the contents of @m@ will not
+ * change, unless @f@ has the @MP_UNDEF@ flag set.
+ *
+ * * If @m@ is not @MP_NEW@, then he reference count of @m@ on
+ * entry is equal to the sum of the counts of @d@ and @m@ on
+ * exit.