/** abcd.java - This is a modular Java program that features functions
 *  (static methods) calling other functions.  Can you figure out the
 *  output?  At any given point, the program needs to know where it is
 *  in its nested function calls.  When a function returns, it should be
 *  no mystery where to return to.  We can draw a function instance graph
 *  to depict the relationship among all function calls.
 */
public class abcd
{
  public static void main(String [] args)
  {
    a();
    System.out.printf("\n");
    b();
    System.out.printf("\n");
    c();
    System.out.printf("\n");
    d();
    System.out.printf("\n");
  }
  
  public static void a()
  {
    System.out.printf("a");
  }
  
  public static void b()
  {
    a();
    System.out.printf("b");
  }
  
  public static void c()
  {
    a();
    b();
    System.out.printf("c");
  }
  
  public static void d()
  {
    a();
    b();
    c();
    System.out.printf("d");
  }
}