The following data set and macro variable contain the additional input data that are used in this example:

/* missing expansion_prerequisite indicates no prerequisite */
data expansion_data;
   input expansion expansion_depot $10. expansion_amount expansion_cost
      expansion_prerequisite;
   datalines;
1 Birmingham 5 18000 .
2 Birmingham 5  8000 1
3 Manchester 5 20000 .
4 Manchester 5  5000 3
5 Plymouth   5 19000 .
;

%let max_num_expansions = 3;