The power “to coin money” and “regulate the value thereof ” has been broadly construed to authorize regulation of every phase of the subject of currency. Congress may charter banks and endow them with the right to issue circulating notes,1465 and it may restrain the circulation of notes not issued under its own authority.1466 To this end it may impose a prohibitive tax upon the circulation of the notes of state banks1467 or of municipal corporations.1468 It may require the surrender of gold coin and of gold certificates in exchange for other currency not redeemable in gold. A plaintiff who sought payment for the gold coin and certificates thus surrendered in an amount measured by the higher market value of gold was denied recovery on the ground that he had not proved that he would suffer any actual loss by being compelled to accept an equivalent amount of other currency.1469 Inasmuch as “every contract for the payment of money, simply, is necessarily subject to the constitutional power of the government over the currency, whatever that power may be, and the obligation of the parties is, therefore, assumed with reference to that power,”1470 the Supreme Court sustained the power of Congress to make Treasury notes legal tender in satisfaction of antecedent debts,1471 and, many years later, to abrogate the clauses in private contracts calling for payment in gold coin, even though such contracts were executed before the legislation was passed.1472 The power to coin money also imports authority to maintain such coinage as a medium of exchange at home, and to forbid its diversion to other uses by defacement, melting or exportation.1473