Section 16. Any  city,  county,  township,  town,  school  district  or  any  other  political 
subdivision incurring indebtedness shall,  at  or  before the time of  so doing,  provide for  the 
collection of an annual tax sufficient to pay the interest and also the principal thereof when 
due, and all laws or ordinances providing for the payment of the interest or principal of any 
debt shall be irrepealable until such debt be paid.