repealable tax exemption

Previous decisions of this court have settled the proposition that whether such exemption has been in fact repealed by a subsequent state statute is a question of state law in which the decisions of the highest courts of the state, in the absence of contract, are binding; and that it is only where the exemption is irrepealable, thus constituting a contract, that it becomes the duty of this court to decide for itself whether the subsequent act did or did not impair the obligation of the contract.