Mura: How to add a validation method to a custom form that doesn't override Mura's async form submission

mura-custom-form.cfm

<script>
Mura(function(m) {
    window.customFormValidation = function(theform) {
        // example validation
        if (theform.somefield.value === '') {
            alert('Please enter a value in somefield');
            theform.somefield.focus();
            return false;
        }

        // let Mura process the form now
        Mura.submitForm(theform);
        return false;
    }
});
</script>

<form method="post" name="someform" onsubmit="window.customFormValidation(this);return false;" novalidate>
    <input name="somefield" />
    <input name="submit" type="submit" value="Submit" />
</form>