Listing 1

Let [x] be a label.

0 procedure tclose([x])
1  if head([x]) is a token
2   then
3    tail    = tclose(tail([x]));
4    closure = [];
5    for t in tail
6     closure = conc(conc(head([x]), t), 
7                    closure);
8    return closure;
9   else
10   closure = [];
11   for h in tclose(head([x]))
12    for t in tclose(tail([x])
13     closure = conc(apnd(h, t), 
14                    closure);
15   return closure;