td>no change</td>
</tr>
<tr>
  <td><b>appearance</b></td>
  <td>many changes &mdash; see below</td>
</tr>
<tr>
  <td>[gravity]</td>
  <td>non-default gravity is now specified in the system model instead of as
    an argument to the dynamics functions</td>
</tr>

<tr><td></td></tr>
<tr><td></td></tr>

<tr>
  <th colspan="2">Planar System Model Data Structure</th>
</tr>
<tr>
  <td><b>NB</b></td>
  <td>no change</td>
</tr>
<tr>
  <td><b>parent</b></td>
  <td>no change</td>
</tr>
<tr>
  <td><b>jcode</b></td>
  