A travelling salesman must visit 7 customers in 7 different locations whose
(symmetric) distance matrix is:

  1  2  3  4  5  6  7
1 - 86 49 57 31 69 50
2    - 68 79 93 24 5
3       - 16  7 72 67
4          - 90 69 1
5             - 86 59
6                - 81

Formulate a mathematical program to determine a visit sequence starting at
ending at location 1, which minimizes the travelled distance, and solve it
with AMPL. Knowing that the distances obey a triangular inequality and are
symmetric, propose a suitable heuristic method.