action player_walk()
   while (1)
   {
// rotate the player using the [A] and [D] keys      
      my.pan += 5*(key_a-key_d)*time_step; 

// determine the ground distance by a downwards trace
      var dist_down; 
      if (c_trace(my.x,vector(my.x,my.y,my.z-5000),IGNORE_ME | IGNORE_PASSABLE | USE_BOX) > 0)
         dist_down = my.z + vFeet.z - target.z; // get distance between player's feet and the ground
      else
         dist_down = 0;

// apply gravity when the player is in the air
      if (dist_down > 0)  // above floor, fall down with increasing speed
         dist_down = clamp(dist_down,0,accelerate(speed_down,5,0.1));
      else                // on or below floor, set downward speed to zero
         speed_down = 0;

// move the player using the [W] and [S] keys      
      var dist_ahead = 5*(key_w-key_s)*time_step;
      dist_ahead = sign(dist_ahead)*(abs(dist_ahead) + 0.5*dist_down); // adapt the speed on slopes
      c_move(me,vector(dist_ahead,0,0),vector(0,0,-dist_down),IGNORE_PASSABLE | GLIDE); // move the player

// animate the player according to its moved distance
      if (dist_ahead != 0) // player is moving ahead
      {
         anim_percent += 1.3*dist_ahead; // 1.3 = walk cycle percentage per quant
         ent_animate(me,"walk",anim_percent,ANM_CYCLE); // play the "walk" animation
      }
      else // player stands still
      { 
         anim_percent += 5*time_step; 
         ent_animate(me,"stand",anim_percent,ANM_CYCLE); // play the "stand" animation
      }
      wait(1);
   }