apply - what is the utility of apply?

Oh, I believe I have found why I don't understand. The template parameters TS are not part of the function apply
This

```
template
constexpr auto apply(F&& f, Ts&&... xs);
```

should be

```
template
constexpr auto apply(F&& f);
```

The semantics given us

```
assert(apply(f)(xs...) == f(xs...));
```

But the usage is incompatible

```
assert(fit::apply(sum_f(), 1, 2) == 3);
```

that I believe should be

``` c££
assert(fit::apply(sum_f())(1, 2) == 3);
```

Unfortunately the implementation is not inline at all.