/*
 * This code is from "Algorithms in C, Third Edition,"
 * by Robert Sedgewick, Addison Wesley Longman, 1998.
 */
void traverse(link h, void (*visit)(link))
  { 
    if (h == NULL) return;
    (*visit)(h); 
    traverse(h->l, visit);
    traverse(h->r, visit);
  }