space;">For option letters in the various
      models, you will often see that there is a default. For instance,
      BELLHOP defaults to a point source in cylindrical coordinates, but
      you can explicitly select a point source ('R') or a line source
      ('X'). If you just want to go with the default, then simply put in
      a blank space for that position.<br>
    </p>
    <p style="font-family: monospace;">The following modules are part of
      the package.</p>
    <h2>Group I: Mode Computations