In Von Hoffman v. City of Quincy (1866) 71 U.S. 535, a subsequent statute attempted to impair contract rights secured under a previous valid statute. Mandamus was applied because "the validity of the bonds was not denied," and the tax levy was authorized by previous statutes. It was said: "A statute of frauds embracing pre-existing parol contracts not before required to be in writing would affect its validity. A statute declaring that the word `ton' should, in prior as well as subsequent contracts, be held to mean half or double the weight before prescribed, would affect its construction. A statute providing that a previous contract of indebtment may be extinguished by a process of bankruptcy would involve its discharge; and a statute forbidding the sale of any of the debtor's property under a judgment upon such a contract would relate to the remedy." In that case, the Supreme Court dealt with incorporation by implication: "It is also settled that the laws which subsist at the time and place of the making of a contract, and where it is to be performed, enter into and form a part of it, as if they were expressly referred to or incorporated in its terms." (Id. at 550.) The Supreme Court said: "It is competent for the States to change the form of the remedy, or to modify it otherwise, as they may see fit, provided no substantial right secured by the contract is thereby impaired. No attempt has been made to fix definitely the line between alterations of the remedy, which are to be deemed legitimate, and those which, under the form of modifying the remedy, impair substantial rights." In the case of Von Hoffman v. City of Quincy, 4 Wall. 535, 554, the Supreme Court of the United States declares a principle of law which we think has a direct bearing on the solution of the issues here involved. The court said: "It is equally clear that where a State has authorized a municipal corporation to contract and to exercise the power of local taxation to the extent necessary to meet its engagements, the power thus given cannot be withdrawn until the contract is satisfied. The State and the corporation in such cases, are equally bound. The power given becomes a trust which the donor cannot annul, and which the donee is bound to execute; and neither the State nor the corporation can any more impair the obligation of the contract in this way than in any other."