def collect_all_methods(cls, method_name):
    """Return list of all `method_name` methods for cls and its superclass chain.

    List is in MRO order, with no duplicates. Methods are unbound.

    (This is used to simplify mixins and subclasses that contribute to a method set,
    without requiring superclass chaining, and without requiring cooperating
    superclasses.)
    """
    methods = []
    for ancestor in cls.__mro__:
        try:
            validator = getattr(ancestor, method_name)
        except AttributeError:
            pass
        else:
            if validator not in methods:
                methods.append(validator)
    return methods