code: function () {
          this.update = {
              gravity: function (entity, dt) {
                  // Accelerate entity until it reaches terminal velocity.
                  if (entity.c.velocity.yspeed < entity.c.gravity.terminal) {
                      entity.c.velocity.yspeed += hitagi.utils.delta(entity.c.gravity.magnitude, dt);
                  }
              }
          };
      }
docs: Define systems.