erlang macro with double question marks

Question

I know that a macro is defined as follows:

-define(Const, Replacement).
-define(Func(Var1,...,VarN), Replacement).

But how does this code with double question marks meam, and how should I understand it?

-define(Assign(Var,Exp), Var=Exp, io:format("~s = ~s -> ~p~n", [??Var, ??Exp, Var] )).

start() ->
   ?Assign(X, lists:sum([1, 2, 3])).

Its output reads:

X = lists : sum ( [ 1 , 2 , 3 ] ) -> 6

Answer 1

After searching, I find it. Stringifying Macro Arguments

The construction ??Arg, where Arg is a macro argument, 
will be expanded to a string containing the tokens of the argument. 
This is similar to the #arg stringifying construction in C.

The feature was added in Erlang 5.0/OTP R7.