Builds the diff dictionary.

def _create_diff(diff, fun, key, prev, curr):

    '''
    Builds the diff dictionary.
    '''

    if not fun(prev):
        _create_diff_action(diff, 'added', key, curr)
    elif fun(prev) and not fun(curr):
        _create_diff_action(diff, 'removed', key, prev)
    elif not fun(curr):
        _create_diff_action(diff, 'updated', key, curr)