// Flier.pj
personality Flier {
  // upstream interface. Must implement here.
  public void Fly(int x, int y, int altitude) {
    resetMetersFlown();
    Takeoff();
    for (int a=0; a < altitude; a++) Ascend();
    while( !ThereYet(x, y) ) FlapTowards(x, y);
    for(int a = altitude; a > 0; a--) Descend();
    Land();
  }
  // downstream interface. Don't impl here.
  di void Takeoff();
  di void Ascend();
  di boolean ThereYet(int x, int y);
  di void FlapTowards(int x, int y);
  di void Descend();
  di void Land();
  // private functions. Must implement here.
  private void resetMetersFlown() { meters_flown = 0; }
  // attributes (specific to the role)
  private float meters_flown;
  // constructor (optional)
  Flier() { resetMetersFlown(); }
}