Added a comment about the unreferenced PyThreadState.tick_counter

member.

Include/pystate.h

@@ -74,7 +74,14 @@ typedef struct _ts {
 
     PyObject *dict;
 
+    /* tick_counter is incremented whenever the check_interval ticker
+     * reaches zero. The purpose is to give a useful measure of the number
+     * of interpreted bytecode instructions in a given thread.  This
+     * extremely lightweight statistic collector may be of interest to
+     * profilers (like psyco.jit()), although nothing in the core uses it.
+     */
     int tick_counter;
+
     int gilstate_counter;
 
     PyObject *async_exc; /* Asynchronous exception to raise */