on lets the
user remove such bricks. This means that they are not going to appear in any
game from here on unless they are added again by selecting <b>Add brick</b>.
This can also be acheived by the keyboard shortcut <b>meta-r</b>.
<li><b>Bricks in use</b><br>
This option lets the user display user defined bricks in play if there are any.
The user is prompted to enter one of the listed brick id's and in doing so,
that brick is displayed in a popup window. The dialog reappears until
<b>Cancel</b> is pressed.