void OnTriggerEnter2D(Collider2D other) {
if (other.IsProjectile ()) {
var projectile = other.GetComponent<Projectile> ();
if (projectile.IsTargetingAlien) {
Score.Instance.Sum (m_alien.gameObject);
m_alien.Die ();
}
}
else if (other.IsCannonZone ()) {
Cannon.Instance.Die ();
}
}