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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 gen_veth_str(self, const_str, intf_str):
 """Generate a veth string. Concatenates the constant string with remaining available length of interface string from trailing position. """

        avl_len = constants.MAX_VETH_NAME - len(const_str)
        if avl_len <= 0:
            LOG.error("veth string name too short")
            raise dfae.DfaAgentFailed(reason="Veth Unavailable")
        start_pos = len(intf_str) - avl_len
        veth_str = const_str + intf_str[start_pos:]
        return veth_str