Build all combinations of strings

["Nordlöw" via Digitalmars-d-learn]

On Sunday, 11 January 2015 at 18:01:09 UTC, bearophile wrote:
>> Is doCopy really needed as an argument here?
>>
>> Couldn't this be inferred from the mutability of T instead?
>
> doCopy is useful, if it's true all the permutation arrays are 
> distinct and dup-ped, otherwise they are all different. It's 
> true by default, so casual users of that generator will avoid 
> bugs. You can set it to false to speed up your code.

Couldn't we do a first pass and check that if elements of T are 
distinct and if so set doCopy to false otherwise true?