Strange function

[Orfeo]

In druntime (core/bitop.d line 302) I found this function

```
/**
  * Tests and resets (sets to 0) the bit.
  */
int btr(size_t* p, size_t bitnum) pure @system;

```



Honestly don't understand: where is the body of the function?

I thought I could find something like that:

```
int btr(size_t* p, size_t bitnum) pure @system {
   // body
}


Thank you