let loopLymphocytes = function () {
    ...

    // update each creature
    let lymphocytes = world.lymphocytes;
    let bacterias = world.bacterias;
    lymphocytes.forEach(function (creature) {
      // move
      let input = [];
      for (let i in bacterias) {
        input.push(bacterias[i].location.x);
        input.push(bacterias[i].location.y);
        input.push(bacterias[i].velocity.x);
        input.push(bacterias[i].velocity.y);
      }
      let output = creature.network.activate(input);
      creature.moveTo(output);

      // learn
      let learningRate = .3;
      let target = [targetXLymphocyte(creature), targetYLymphocyte(creature), targetAngleLymphocyte(creature)];
      creature.network.propagate(learningRate, target);

      // draw
      creature.draw();
    });
    if (true)
      setTimeout(loopLymphocytes, 1000 / fps);
  }