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              var a = {
  // the current desired steering 
  steering: new vec2(),
  // for random walk, current turning tendency
  wander: vec2.random(),  
  // clipping:
  max_force: 1/2000,
  max_speed: 1/200,
  // physics:
  pos: new vec2(0.5, 0.5),
  vel: vec2.random(0.001),
  // visual:
  size: 1/40

// simple locomotion model
function locomotion(a) {
  // interpret steering as a force to get acceleration
  var force = a.steering.limit(a.max_force);
  var acceleration = force; // divided by mass?
  // forward Euler integration & clipping
  // keep in world

// not recommended!
function steering_twitchy_wander(a) {
  a.steering = vec2.random();

function steering_wander(a) {
  // Reynold's recommendation:
  // random deviations to a stored value
  // constrained to lie on a circle
  // put this directly ahead of the agent:
  var desired_velocity = a.vel.clone().len(0.5).add(a.wander);
  // usual stuff:
  a.steering = desired_velocity.sub(a.vel);

function update() {
function draw() {
  // push into agent's local coordinate system
      // draw agent body:
      // draw agent eyes:
      draw2D.circle([0.5, 0.5], 0.5);
      draw2D.circle([0.5, -0.5], 0.5);
    // done drawing agent:
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