The order of operations in
`````````````````
list_head.compare_exchange_weak( oldHead, oldHead->next ) // part of pop
`````````````````
This is a function call, before the actual call (which will be atomic) all arguments have to be evaluated,
thus

1. evaluate `oldHead->next ` (will be placed in some temporary memory)
2. call     `compare_exchange_weak ` 

Thread may be interrupted between 1) and 2)

Example of ABA:
---------------

Original list:
************************************************
*          20        30      40       50       *
*        +-----+  +-----+  +-----+  +-----+    *
*        |     +->|     +->|     +->|     +->  *  
*        +-----+  +-----+  +-----+  +-----+    *
*           ^                                  *
*  head     |                                  *
*  +-----+  |                                  *
*  | 20  +--+                                  *
*  +-----+                                     *
************************************************

T1 start a pop:
````````````
location  = head
old state = 20
desired   = 30  // next of the current head -- see note above
````````````

T2 kicks in: 
T2 pop
**************************************************
*            30      40       50                 *
*         +-----+  +-----+  +-----+              *
*         |     +->|     +->|     +->            *
*         +-----+  +-----+  +-----+              *
*            ^                                   *
*  head      |                                   *
*  +-----+   |                                   *
*  | 30  +---+                                   *
*  +-----+                                       *
**************************************************

T2 push
**************************************************
*            60       30      40       50        *
*         +-----+  +-----+  +-----+  +-----+     *
*         |     +->|     +->|     +->|     +->   * 
*         +-----+  +-----+  +-----+  +-----+     *
*            ^                                   *
*  head      |                                   *
*  +-----+   |                                   *
*  | 60  +---+                                   *
*  +-----+                                       *
**************************************************

T2 push (this push reuses address 20)
***********************************************************
*            20       60       30      40       50        *
*         +-----+  +-----+  +-----+  +-----+  +-----+     *
*         |     +->|     +->|     +->|     +->|     +->   * 
*         +-----+  +-----+  +-----+  +-----+  +-----+     *
*            ^                                            *
*head        |                                            *
*+-----+     |                                            *
*| 20  +-----+                                            *
*+-----+                                                  *
***********************************************************

T1 continues:
``````````
location == old state // true - success
head = desired = 30 !!!!!
``````````
*************************************************************
*            60       30      40       50                   *
*         +-----+  +-----+  +-----+  +-----+                *
*         |     +->|     +->|     +->|     +->              *
*         +-----+  +-----+  +-----+  +-----+                *
*                     ^                                     *
*  head               |                                     *
*  +-----+            |                                     *
*  | 30  |------------+                                     *
*  +-----+                                                  *
*************************************************************

**Two-headed** linked list!



Another example of ABA:
-----------------------

Original list:
*****************************************************
*           20        30      40       50           *
*         +-----+  +-----+  +-----+  +-----+        *
*         |     +->|     +->|     +->|     +->      *
*         +-----+  +-----+  +-----+  +-----+        *
*            ^                                      *
*  head      |                                      *
*  +-----+   |                                      *
*  | 20  +---+                                      *
*  +-----+                                          *
*****************************************************

T1 start a pop:
location  = head
old state = 20
desired   = 30  (next of the current head)

T2 kicks in: 

T2 pop
******************************************************
*            30      40       50                     *
*         +-----+  +-----+  +-----+                  *
*         |     +->|     +->|     +->                *
*         +-----+  +-----+  +-----+                  *
*            ^                                       *
*  head      |                                       *
*  +-----+   |                                       *
*  | 30  +---+                                       *
*  +-----+                                           *
******************************************************

T2 pop
******************************************************
*           40       50                              *
*         +-----+  +-----+                           *
*         |     +->|     +->                         *
*         +-----+  +-----+                           *
*            ^                                       *
*  head      |                                       *
*  +-----+   |                                       *
*  | 40  +---+                                       *
*  +-----+                                           *
******************************************************

T2 push (this push reuses address 20)
******************************************************
*            20      40       50                     *
*         +-----+  +-----+  +-----+                  *
*         |     +->|     +->|     +->                *
*         +-----+  +-----+  +-----+                  *
*            ^                                       *
*  head      |                                       *
*  +-----+   |                                       *
*  | 20  +---+                                       *
*  +-----+                                           *
******************************************************

T1 continues:
`````````````````
location == old state // true - success
head = desired = 30 !!!!!
`````````````````
******************************************************
*            60      40       50                     *
*         +-----+  +-----+  +-----+                  *
*         |     +->|     +->|     +->                *
*         +-----+  +-----+  +-----+                  *
*                                                    *
*head                                                *
*+-----+                                             *
*| 30  |--------------> points to deleted memory     *
*+-----+                                             *
******************************************************


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