Among others, LENGTH (FILTER (\e. X = e)) = 1

Scott

> On 2017/10/05, at 15:55, Chun Tian (binghe) <[email protected]> wrote:
> 
> But my list is not ALL_DISTINCT … there’re duplicated elements for sure, just 
> the appearance of certain element is only one. —Chun
> 
>> Il giorno 05 ott 2017, alle ore 16:48, Konrad Slind <[email protected]> 
>> ha scritto:
>> 
>> Hi Chun Tian,
>> 
>>  The constant ALL_DISTINCT in listTheory is what you are looking for, I 
>> think.
>> 
>> Konrad.
>> 
>> 
>> On Thu, Oct 5, 2017 at 9:03 AM, Chun Tian (binghe) <[email protected]> 
>> wrote:
>> Hi,
>> 
>> (I'm new to HOL's listTheory). Suppose I have a list L which contains 
>> possibly duplicated elements, and I want to express certain element X is 
>> unique (i.e. has only one copy) in that list.  How can I do this?  It seems 
>> "?!" doesn't work here, e.g. ``?!e. MEM e L /\ (e = X)``.
>> 
>> Regards,
>> 
>> Chun Tian
>> 
>> --
>> Chun Tian (binghe)
>> University of Bologna (Italy)
>> 
>> 
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