Clarify the relationship between indices.
diff --git a/Modules/collectionsmodule.c b/Modules/collectionsmodule.c
index 188b239..27236d9 100644
--- a/Modules/collectionsmodule.c
+++ b/Modules/collectionsmodule.c
@@ -7,7 +7,7 @@
All rights reserved.
*/
-#define BLOCKLEN 46
+#define BLOCKLEN 2
#define CENTER ((BLOCKLEN - 1) / 2)
/* A `dequeobject` is composed of a doubly-linked list of `block` nodes.
@@ -24,17 +24,19 @@
*
* The indices, d.leftindex and d.rightindex are always in the range
* 0 <= index < BLOCKLEN.
+ * Their exact relationship is:
+ * (d.leftindex + d.len - 1) % BLOCKLEN == d.rightindex.
*
* Empty deques have d.len == 0; d.leftblock==d.rightblock;
* d.leftindex == CENTER+1; and d.rightindex == CENTER.
* Checking for d.len == 0 is the intended way to see whether d is empty.
*
* Whenever d.leftblock == d.rightblock,
- * d.leftindex + d.len == d.rightindex + 1.
+ * d.leftindex + d.len - 1 == d.rightindex.
*
- * However, when d.leftblock != rightblock, d.leftindex and d.rightindex
- * are indices into distinct blocks and have no relationship to one
- * another (for example, sometimes d.leftindex > d.rightindex).
+ * However, when d.leftblock != d.rightblock, d.leftindex and d.rightindex
+ * become indices into distinct blocks and either may be larger than the
+ * other.
*/
typedef struct BLOCK {