How can I express the type of a function in D?

[Sobaya]

I want to get a mangled name of a D function by 
`core.demangle.mangle`, but I'm in trouble because there are no 
ways to express a type of a function, which is used for a 
template argument of `mangle`.

For example, it is wrong to use the type `int function(int,int)` 
to express the type of `int add(int,int)`.
Because it expresses the type of a function POINTER, not just a 
function.

The fuction name in a binary compiled this function is 
"_D3addFiiZi", but `mangle!(int function(int,int))("add")` 
returns "_D3addPFiiZi", which includes "P" meaning POINTER.

How can I get the former one?

Thanks.