What is this function call operator?

[Adam D. Ruppe via Digitalmars-d-learn]

On Thursday, 27 August 2015 at 16:03:58 UTC, Gary Willoughby 
wrote:
> Sorry, I mean the three dots '...' that seems to be what the 
> documentation is referring to. Also the `isCallable` template 
> uses it.

That just means it can take whatever arguments. The documentation 
is just saying it is callable with anything; the ... is a 
placeholder. In the template itself, (T...) is a variadic 
argument list, again meaning it can take anything. (The reason 
isCallable does it though isn't to actually take multiple 
arguments, it is so it can take any *kind* of argument; a bit of 
a hack.)