In this form of animation, you can specify objects that have independently
controllable velocities and accelerations in the X and Y directions.
Components can still sequence.

The variables that control this motion are located in the animob structure
and are called:

  * yvel, xvel--the velocities in the y and x directions. these
    values are added to the position values on each call to animate()
    (see below).

  * yaccel, xaccel--the accelerations in the xa nd y directions.
    These values are added to the velocity values on each call to
    animate() (see below).  the velocity values are updated before the
    position values.