How to use pure in D 2.0 question on Stack Overflow?

[Walter Bright]

BCS wrote:
> Reply to Jeroen,
> 
>> Does anyone know the answer to this D related question on SO?
>>
>> http://stackoverflow.com/questions/1008803/how-to-use-pure-in-d-2-0/
>>
> 
> DMD can't tell that the method call is safe even though it is inpure.

Also, the purity of a function is determined by its signature, not by 
its implementation.