/*
 * This code is from "Algorithms in C, Third Edition,"
 * by Robert Sedgewick, Addison Wesley Longman, 1998.
 */
link STjoin(link a, link b)
  { 
    if (b == z) return a;
    if (a == z) return b;
    b = STinsert(b, a->item); 
    b->l = STjoin(a->l, b->l); 
    b->r = STjoin(a->r, b->r); 
    free(a);    
    return b;
  }