Strange expression found in std.variant

[Stefan Koch via Digitalmars-d-learn]

On Saturday, 3 June 2017 at 16:22:33 UTC, Francis Nixon wrote:
> When looking at std.variant I found the following line:
> return q{
>     static if (allowed!%1$s && T.allowed!%1$s)
>          if (convertsTo!%1$s && other.convertsTo!%1$s)
>              return VariantN(get!%1$s %2$s other.get!%1$s);
> }.format(tp, op);
>
> I was wondering what exactly the % signs where doing/what they 
> are for?

Those are positional arguments to format.
%1$s refers to the first argument (in this case tp)
while  %2$s refers to the second (op)