model:
sets:
nodes/a1,a2,a3,a4,a5,a6/; 
lines(nodes,nodes)/
a1,a2 a1,a3 a1,a4 a1,a5 a1,a6 a2,a3 a2,a4 a2,a5
a2,a6 a3,a4 a3,a5 a3,a6 a4,a5 a4,a6 a5,a6/:p,x; 
endsets
data:
p=15 20 27 37 54 15 20 27 
37 16 21 28 16 21 17;
enddata
n=@size(nodes);
min=@sum(lines:p*x);
@for(nodes(i)|i#ne#1#and#i#ne#n:
@sum(lines(i,j):x(i,j))=@sum(lines(j,i):x(j,i)));
@sum(lines(i,j)|i#eq#1:x(i,j))=1;
@sum(lines(i,j)|j#eq#6:x(i,j))=1;
