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// athlete.js
function Athlete(name) {
  this.name = name;
}

Athlete.prototype.attack = function(opponent) {
  print(this.name + ' attacked ' + opponent.name);
};

// footballer.js
function Footballer(name, position) {
  Athlete.call(name);
  this.position = position;
}
Footballer.extend(Athlete);

Footballer.prototype.kick = function() {
  print(this.name + ' shoots...');

  var self = this;
  this.shoot({
    onSuccess: function() { print(self.name + ' scores!'); }, 
    onFailure: function() { print('Juuust a bit outside.'); } 
  });
};

Footballer.prototype.shoot(callbacks) {
  // ...
}

// defender.js
function Defender(name) {
  Footballer.call(name, 'defender');
}
Defender.extend(Footballer);

Defender.prototype.attack = function() {
  this.__super();

  this.slideTackle();
  this.feignInjury();
};

Defender.prototype.slideTackle = function() {
  // ...
};

Defender.prototype.feignInjury = function() {
  // ...
};