§ 131. Letting of contract to be public. Every such contract shall be
let at public letting as required by law, and as a consideration for the
execution and performance thereof, shall expressly provide for and
secure to such city, prices lower than any now prescribed therein by
law, and a progressive reduction in such price for each year during the
continued performance of such contract and also adequate assurance of
the continuing mutual performance of such contract according to the
conditions thereof, with proper indemnity to either party to such
contract against any possible violation, impairment, abrogation or
supersession thereof, within the term specified.