ngrx 4 testing effects jasmine-marbles

I am working on testing effects in ngrx store. And after updating ngrx to version I've found that the way, how to test it, has been changed.

import { hot, cold } from 'jasmine-marbles';

it('should work', () => {
   actions = hot('--a-', { a: SomeAction });

   const expected = cold('--b', { b: AnotherAction });

   expect(effects.someSource$).toBeObservable(expected);
});  

Unfortunately I didn't find any documentations for jasmine-marbles. Could anybody explain what mean the first parameter for hot an cold functions? And is it possible to find any documentation about it?