Document the purpose of the struct _block members.
diff --git a/Python/pyarena.c b/Python/pyarena.c
index 33261b4..242ca1d 100644
--- a/Python/pyarena.c
+++ b/Python/pyarena.c
@@ -1,7 +1,7 @@
#include "Python.h"
#include "pyarena.h"
-/* A simple arena block structure
+/* A simple arena block structure.
Measurements with standard library modules suggest the average
allocation is about 20 bytes and that most compiles use a single
@@ -10,9 +10,26 @@
#define DEFAULT_BLOCK_SIZE 8192
typedef struct _block {
+ /* Total number of bytes owned by this block available to pass out.
+ * Read-only after initialization. The first such byte starts at
+ * ab_mem.
+ */
size_t ab_size;
+
+ /* Total number of bytes already passed out. The next byte available
+ * to pass out starts at ab_mem + ab_offset.
+ */
size_t ab_offset;
+
+ /* An arena maintains a singly-linked, NULL-terminated list of
+ * all blocks owned by the arena. These are linked via the
+ * ab_next member.
+ */
struct _block *ab_next;
+
+ /* Pointer to the first allocatable byte owned by this block. Read-
+ * only after initialization.
+ */
void *ab_mem;
} block;