how to define my own traits

[XavierAP via Digitalmars-d-learn]

I've looked into Phobos to emulate it when defining my own trait 
template, and when I see this:

module std.range.primitives;
// ...
template isInputRange(R)
{
     enum bool isInputRange = is(typeof(
     (inout int = 0)
     {
         R r = R.init;     // can define a range object
         if (r.empty) {}   // can test for empty
         r.popFront;       // can invoke popFront()
         auto h = r.front; // can get the front of the range
     }));

I wonder, why that unused parameter (inout int = 0)?
In my project () { /* ... */ } works the same for a custom trait.