SF# 1409443: Expand comment to cover the interaction between f->f_lasti and the PREDICT macros.

Author: rhettinger

Python/ceval.c

@@ -741,7 +741,16 @@ PyEval_EvalFrameEx(PyFrameObject *f, int throwflag)
 	   this wasn't always true before 2.3!  PyFrame_New now sets
 	   f->f_lasti to -1 (i.e. the index *before* the first instruction)
 	   and YIELD_VALUE doesn't fiddle with f_lasti any more.  So this
-	   does work.  Promise. */
+	   does work.  Promise. 
+
+	   When the PREDICT() macros are enabled, some opcode pairs follow in
+           direct succession without updating f->f_lasti.  A successful 
+           prediction effectively links the two codes together as if they
+           were a single new opcode; accordingly,f->f_lasti will point to
+           the first code in the pair (for instance, GET_ITER followed by
+           FOR_ITER is effectively a single opcode and f->f_lasti will point
+           at to the beginning of the combined pair.)
+	*/
 	next_instr = first_instr + f->f_lasti + 1;
 	stack_pointer = f->f_stacktop;
 	assert(stack_pointer != NULL);