validation

def validate_placeholder_refs(tree):
    allowed = set([k for (k, v) in tree['template-set.json']['placeholders'].items()])

    def check_refs(placeholder):
        for k, v in placeholder.items():
            if not k in allowed:
                raise Invalid('no-such-placeholder', path=[k])
        return placeholder

    schema = Schema({
        Match(REGEX_WIDTH_HEIGHT): {
            'template.json': {
                'placeholderExceptions': check_refs
            }
        }
    }, extra=ALLOW_EXTRA)
    return schema(tree)