I'm working on a form where I'd like to have some custom validation rules like: 

> 
> field 2 has to be larger than field 1
> 
> 
> field 3 is required if field 2 is not negative
> 
> 
> ...
> 
> 
> 

**HTML** 

```
<table>
<tr><th>Category</th><th> > </th> ≤ <th></tr>

<tr><td>Green</td>
    <td><input type="number" id="field1" ng-model="green.low"/></td>
    <td><input type="number" id="field2" ng-model="green.high"/></td></tr>
</table>

```

**JS**

I check the validation this way:

```
function verifyForm(form, scope) {
    if (form.$error.required) {
        scope.addAlert("danger", "[![base.error.msg.required_fields]]");
        return false;
    }
    if (!form.$valid) {
        scope.addAlert("danger", "[![base.error.msg.invalid_form]]");
        return false;
    }
    return true;
};

```

So when the submit button is clicked, I just have to do this:

```
if (!verifyForm($scope.formName, $scope))
        return;

```

How can I add those custom validation rules? Or, if I have to write them myself in javascript, how can I "invalidate" certain elements?