lingo问题求教

mkl

用lingo求解最优连线问题时，代码如下，麻烦问一下红色标出的什么意思
sets:
  cities/1..10/:level; !level(i)= the level of city;            
  link(cities, cities):
     distance, !The distance matrix;
     x;        ! x(i,j)=1 if we use link i,j;
endsets
data:  !Distance matrix, it need not be symmetirc;                 
  distance = 0  8  5  9 12 14 12 16 17 22
             8  0  9 15 16  8 11 18 14 22
             5  9  0  7  9 11  7 12 12 17
             9 15  7  0  3 17 10  7 15 15
            12 16  9  3  0  8 10  6 15 15
            14  8 11 17  8  0  9 14  8 16   
            12 11  7 10 10  9  0  8  6 11
            16 18 12  7  6 14  8  0 11 11
            17 14 12 15 15  8  6 11  0 10
            22 22 17 15 15 16 11 11 10  0;
enddata
n=@size(cities); !The model size;
! Minimize total distance of the links;
min=@sum(link(i,j)|i #ne# j: distance(i,j)*x(i,j));
!There must be an arc out of city 1;
@sum(cities(i)|i #gt# 1: x(1,i))>=1;
!For city i, except the base (city 1);
@for(cities(i) | i #gt# 1 :
!  It must be entered;
   @sum(cities(j)| j #ne# i: x(j,i))=1;
!  level(j)=levle(i)+1, if we link j and i;
   @for(cities(j)| j #gt# 1 #and# j #ne# i :
      level(j) >= level(i) + x(i,j)
               - (n-2)*(1-x(i,j)) + (n-3)*x(j,i);
   );
!  The level of city is at least 1 but no more n-1,
   and is 1 if it links to base (city 1);
   @bnd(1,level(i),999999);
   level(i)<=n-1-(n-2)*x(1,i);
);
! Make the x's 0/1;
@for(link : @bin(x));