$title using dynamic sets to find roots and leaves of a graph

* graph has seven nodes
set i/1*7/;
alias (i,j);

* define (directed) arc set
set arc(i,j)/1.2, 1.3, 3.4, 2.5, 4.5, 5.6, 5.7/;

* define sets or leaf nodes and root nodes
set leaves(i), roots(i);

* define the leaves to be those nodes that are not the start
* node for an arc.
leaves(i) = yes;
loop(arc(i,j),
  leaves(i) = no;
);

* define the roots to be those that are never the finish 
* node for an arc
roots(i) = yes;
loop(arc(i,j),
  roots(j) = no;
);

display leaves, roots;