bug#27530: patch to cut and copy secondary

[Tak Kunihiro]

>> The function `mouse-exchange-point-and-mark-secondary' exchanges `mark
>> and point' and secondary.  When there is neither primary nor
>> secondary, it behaves as `exchange-point-and-mark'.  By doing this, no
>> revision is necessary for functions that react to region.
> 
> This description seems to contradict the docstring:
>>  
>> +(defun mouse-exchange-point-and-mark-secondary (&optional arg)
>> +  "Exchange the point and the mark, and the secondary selection.
>> +When the mark is active, this exchanges the point and the mark
>> +then creates the secondary selection from the primary selection.
>> +When the mark is not active but the secondary selection exists,
>> +this restores the primary selection from the secondary selection."
>         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> 
> Could you explain in more detail how this functions? The behaviour
> seems useful, but I don't understand exactly what it does just from
> the docstring.

The function exchanges point and mark.
When there is region, this also creates secondary.
When there is secondary, this converts secondary to primary in advance.

As a consequence, an user sees the function exchanges
 - `point' and `mark'
 - `point and mark' (= primary) and secondary
at the same time.