`.combineLatest` allows you to monitor the state of multiple observables at once compactly at a single location. The example demonstrated shows how you can use `.combineLatest` to validate a basic form. There are 3 primary inputs for this form to be considered "valid" (an email, a password and a number). The form will turn valid (the text below turns blue :P) once all the inputs are valid. If they are not, an error is shown against the invalid inputs.