Tuple!(string, int))[] field_orders

[Ozan via Digitalmars-d-learn]

Hi
In vibe.d / data / mongo / collection  I found the function
* ensureIndex(Tuple!(string, int))[] field_orders)
What could be the right way to use "Tuple!(string, int))[] 
field_orders"?
I tried different ways like [Tuple!("a", 1), Tuple!("b", 2)]", 
but compiler said "No"

Any hint?

Thanks & Regards,
Ozan