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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def as_event_class(obj):
 """ Convert obj into a subclass of AbinitEvent. obj can be either a class or a string with the class name or the YAML tag """

    if is_string(obj):
        for c in all_subclasses(AbinitEvent):
            if c.__name__ == obj or c.yaml_tag == obj: return c
        raise ValueError("Cannot find event class associated to %s" % obj)

    # Assume class.
    assert obj in all_subclasses(AbinitEvent)
    return obj