typedef struct _greenlet {
PyObject_HEAD
char* stack_start;
char* stack_stop;
char* stack_copy;
intptr_t stack_saved;
struct _greenlet* stack_prev;
struct _greenlet* parent;
PyObject* run_info;
struct _frame* top_frame;
int recursion_depth;
PyObject* weakreflist;
PyObject* exc_type;
PyObject* exc_value;
PyObject* exc_traceback;
PyObject* dict;
} PyGreenlet;

A PyGreenlet is a range of C stack addresses that must be
saved and restored in such a way that the full range of the
stack contains valid data when we switch to it.
Stack layout for a greenlet:
 | ^^^ |
 | older data |
 | |
 stack_stop . |_______________|
 . | |
 . | greenlet data |
 . | in stack |
 . * |_______________| . . _____________ stack_copy + stack_saved
 . | | | |
 . | data | |greenlet data|
 . | unrelated | | saved |
 . | to | | in heap |
 stack_start . | this | . . |_____________| stack_copy
 | greenlet |
 | |
 | newer data |
 | vvv |

from greenlet import greenlet
def test1():
 print 12
 gr2.switch()
 print 34
def test2():
 print 56
 gr1.switch()
 print 78
gr1 = greenlet(test1)
gr2 = greenlet(test2)
gr1.switch()