From the newsgroup: ``Something is engraved here on the water.''
An engraving written on a lowered drawbridge would be transfered to the
underlying terrain when the bridge was raised. The coverse was also true;
if you plugged the moat and then engraved on the ground, that engraving
would remain visible--and touchable if blind--when the bridge was lowered
to cover its spot. Rather than try to handle two different engravings at
the same coordinates (with one of those two changing locations when the
bridge is raised or lowered), this just wipes out any engraving at both
span and portcullis locations whenever the bridge state changes, like is
done for traps.
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