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+#ifndef ITERATOR_H
+#define ITERATOR_H
+
+/*
+ * Generic constants related to iterators.
+ */
+
+/*
+ * The attempt to advance the iterator was successful; the iterator
+ * reflects the new current entry.
+ */
+#define ITER_OK 0
+
+/*
+ * The iterator is exhausted and has been freed.
+ */
+#define ITER_DONE -1
+
+/*
+ * The iterator experienced an error. The iteration has been aborted
+ * and the iterator has been freed.
+ */
+#define ITER_ERROR -2
+
+/*
+ * Return values for selector functions for merge iterators. The
+ * numerical values of these constants are important and must be
+ * compatible with ITER_DONE and ITER_ERROR.
+ */
+enum iterator_selection {
+	/* End the iteration without an error: */
+	ITER_SELECT_DONE = ITER_DONE,
+
+	/* Report an error and abort the iteration: */
+	ITER_SELECT_ERROR = ITER_ERROR,
+
+	/*
+	 * The next group of constants are masks that are useful
+	 * mainly internally.
+	 */
+
+	/* The LSB selects whether iter0/iter1 is the "current" iterator: */
+	ITER_CURRENT_SELECTION_MASK = 0x01,
+
+	/* iter0 is the "current" iterator this round: */
+	ITER_CURRENT_SELECTION_0 = 0x00,
+
+	/* iter1 is the "current" iterator this round: */
+	ITER_CURRENT_SELECTION_1 = 0x01,
+
+	/* Yield the value from the current iterator? */
+	ITER_YIELD_CURRENT = 0x02,
+
+	/* Discard the value from the secondary iterator? */
+	ITER_SKIP_SECONDARY = 0x04,
+
+	/*
+	 * The constants that a selector function should usually
+	 * return.
+	 */
+
+	/* Yield the value from iter0: */
+	ITER_SELECT_0 = ITER_CURRENT_SELECTION_0 | ITER_YIELD_CURRENT,
+
+	/* Yield the value from iter0 and discard the one from iter1: */
+	ITER_SELECT_0_SKIP_1 = ITER_SELECT_0 | ITER_SKIP_SECONDARY,
+
+	/* Discard the value from iter0 without yielding anything this round: */
+	ITER_SKIP_0 = ITER_CURRENT_SELECTION_1 | ITER_SKIP_SECONDARY,
+
+	/* Yield the value from iter1: */
+	ITER_SELECT_1 = ITER_CURRENT_SELECTION_1 | ITER_YIELD_CURRENT,
+
+	/* Yield the value from iter1 and discard the one from iter0: */
+	ITER_SELECT_1_SKIP_0 = ITER_SELECT_1 | ITER_SKIP_SECONDARY,
+
+	/* Discard the value from iter1 without yielding anything this round: */
+	ITER_SKIP_1 = ITER_CURRENT_SELECTION_0 | ITER_SKIP_SECONDARY
+};
+
+#endif /* ITERATOR_H */