I'm curious to know if anyone has written a procedure to optimize the VRP with time windows / constraints.

I am working to minimize an objective function concerning a facility location problem (p-median technique).

Can somebody explain how to formulate the constraint equation that involves  1 or 0?

For instance, the constraint x[i,j]  is an element of {0,1}; namely, x[i,j] = 1 if a point is assigned to a facility located at the point j; 0 otherwise.

Thanks in advance.