* ``compare``. Compares attribute being validated with some other attribute
value. Supported comparison operations include: ``equal``,
``not_equal``. See :py:class:`~wheezy.validation.rules.CompareRule`.
+* ``predicate``. Fails if predicate return boolean ``False``. Predicate
+ is any callable that accepts model and returns boolean. It is
+ useful for custom rules, e.g. a number of days between two model
+ properties must not exceed certain value, etc.
+ See :py:class:`~wheezy.validation.rules.PredicateRule`.
* ``regex``. Search for regular expression pattern. Initialized with
``regex`` as a regular expression pattern or a pre-compiled regular
expression. Supports ``nagated`` argument.