Assert example does not compile

[clacladev]

I copied and pasted the content of the assert example in a file called assert.cairo:

fn main(x: felt252, y: felt252) {
    assert(x != y, 'error, x is equal to y');
}

#[test]
fn test_main() {
    main(1,2);
}

Then compiling with the suggested command fails:

> cairo-test assert.cairo
Error: The given path is a file, but --single-file was not supplied.

So I compile with the suggested flag, but I get a ton of errors similar to this one:

> cairo-test --single-file assert.cairo
error: Expected function, found module.
 --> assert.cairo:2:5
    assert(x != y, 'error: x is equal to y');
    ^****^

Any ideas why?