I'm working with some Specs tests and I'm trying to understand what the "must" function is, and what it does.

I am unable to find its declaration or implementation anywhere in the specs source, and I'm trying to understand what it does.

Here are some example usages of it:

```
"hello world".size must be equalTo(11)
"hello world" must be matching("h.* w.*")
stack.push(11) must throwAn[Error]

```

It looks to me like "must" takes a function as an argument, but I'd like to know the actual signature of "must", and what it does with its argument.

Can anyone point me in the right direction?

Thanks!