+ * `ARGPAR_ITER_NEXT_STATUS_ERROR_UNKNOWN_OPT`:
+ * Unknown option (not found in `descrs` as passed to
+ * argpar_iter_create() to create `iter`).
+ *
+ * `ARGPAR_ITER_NEXT_STATUS_ERROR_MISSING_OPT_ARG`:
+ * Missing option argument.
+ *
+ * `ARGPAR_ITER_NEXT_STATUS_ERROR_UNEXPECTED_OPT_ARG`:
+ * Unexpected option argument.
+ *
+ * `ARGPAR_ITER_NEXT_STATUS_ERROR_MEMORY`:
+ * Memory error.
+ *
+ * * Except for the `ARGPAR_ITER_NEXT_STATUS_ERROR_MEMORY` status,
+ * sets `*error`, if not `NULL`, to a descriptive error string.
+ * Free `*error` with free().