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  Traveling Salesman Problem variation,
  Given N cities, and a parameter NUM_STOPS <= N,
  Find the shortest tour that 
   visits NUM_STOPS cities exactly once.
 This implementation uses the subtour elimination method, i.e.,
  Solve initially with just the assignment constraints.
  WHILE( solution has subtours:
    Add cut(s) to cut off subtours.
    Solve the new problem with new subtour cuts.
       );

For prize?collecting TSP mode, set NUM_STOPS to the number of cities
 you want the salesperson to visit. A real?world example is Chicago’s 
 Independent Bookstore Crawl, where participants only need to visit 10
 out of 80+ bookstores to earn a 10% discount for the entire year.;

 Ref:  Dantzig, G., R. Fulkerson, and S. Johnson (1954),
  "Solution of a Large-Scale Traveling-Salesman Problem",
  Journal of the Operations Research Society of America,
  Vol. 2, No. 4, November, pp. 393-410;