Sets / associative arrays with void values?

[Vladimir Panteleev]

I've been wondering whether D supported sets, or something similar. What I  
have in mind isn't unlike associative arrays, but without values: an  
unordered list with quick look-up and simple addition/removal of elements,  
yet still capable of enumeration, etc.

I followed my intuition and tried to declare an associative array with a  
"void" value type:

void[int] foo;

To my surprise, it compiled. And even the "in" operation and the .keys and  
.values properties worked fine (or as fine as they can work with an empty  
associative array).

Of course, the question comes: how would one add keys to such a construct?  
The practical answer is obvious: use a real "dummy" data type (bit, for  
example) as the value type, but I find it a shame that declaring such  
constructs is allowed - yet you can't really use them.

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