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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 resolve_ref(cls, domain, env, fromdocname, builder, typ, target, node, contnode):
 """ Resolve a reference to a directive of this class """

        targets = domain.data['targets'].get(cls.ref_type, {})
        try:
            todocname, targetname = targets[target]
        except KeyError:
            env.warn(fromdocname, "Missing BB reference: bb:%s:%s" % (cls.ref_type, target),
                     node.line)
            return None

        return make_refnode(builder, fromdocname,
                            todocname, targetname,
                            contnode, target)