IMPORT Ct := Co_Tasks, Co := Co_Routines, Log;
   
   TYPE
      Tree = POINTER TO TreeDesc;
      TreeDesc = RECORD
         name: ARRAY 16 OF CHAR;
         left, right: Tree
      END;
      
      Traverse = POINTER TO TraverseDesc;
      TraverseDesc = RECORD (Co.CoroutineDesc)
         tree, leaf: Tree
      END;
      
   PROCEDURE NextTreeLeaves (trav: Traverse; tree: Tree);
   BEGIN
      IF (tree.left = NIL) & (tree.right = NIL) THEN
         trav.leaf := tree;
         Co.Yield
      END;
      IF tree.left # NIL THEN
         NextTreeLeaves (trav, tree.left)
      END;
      IF tree.right # NIL THEN
         NextTreeLeaves (trav, tree.right)
      END;
   END NextTreeLeaves;
      
   PROCEDURE Do (t: Ct.Task);
      VAR trav: Traverse;
   BEGIN
      trav := t(Traverse);
      NextTreeLeaves(trav, trav.tree);
      Co.Stop
   END Do;
   
   PROCEDURE Problem*;
      VAR t1, t2, c1, c2: Tree; r1, r2: Traverse; same: BOOLEAN;
   BEGIN
      (* two trees are different, but have the same fringe: a, b, c *)
      NEW(t1); NEW(c1); t1.left := c1; NEW(c2); c2.name := "a"; c1.left := c2; NEW(c2); c2.name := "b";
      c1.right := c2; NEW(c1); t1.right := c1; NEW(c2); c2.name := "c"; c1.left := c2;
      NEW(t2); NEW(c1); t2.left := c1; NEW(c2); c2.name := "a"; c1.left := c2; NEW(c2); c2.name := "b";
      c1.right := c2; NEW(c1); t2.right := c1; NEW(c2); c1.left := c2;
      NEW(c1); c1.name := "c"; c2.left := c1;
      
      NEW(r1); r1.tree := t1;
      NEW(r2); r2.tree := t2;
      same := TRUE;
      Co.InitCoroutine(r1, Do); Co.InitCoroutine(r2, Do);
      r1.Start(r1); r2.Start(r2);
      WHILE ~r1.eor & ~r2.eor & same DO
         r1.Transfer(r1);
         r2.Transfer(r2);
         same := (r1.leaf.name$ = r2.leaf.name$)
      END;
      IF ~r1.eor OR ~r2.eor THEN same := FALSE END;
      Log.String("tree1(a,b,c) vs tree2(a,b,c): The Same Fringe is"); Log.Bool(same); Log.Ln;
      
      (* modify second, append 'd' *)
      NEW(c2); c2.name := "d"; c1.right := c2;
      same := TRUE;
      Co.InitCoroutine(r1, Do); Co.InitCoroutine(r2, Do);
      r1.Start(r1); r2.Start(r2);
      WHILE ~r1.eor & ~r2.eor & same DO
         r1.Transfer(r1);
         r2.Transfer(r2);
         same := (r1.leaf.name$ = r2.leaf.name$)
      END;
      IF ~r1.eor OR ~r2.eor THEN same := FALSE END;
      Log.String("tree1(a,b,c) vs tree2(a,b,c,d) The Same Fringe is"); Log.Bool(same); Log.Ln;
      
   END Problem;