Map_cache
Resets all cache data structures. To speed up map access, each record that is read from a map file is cached into memory so that it can more quickly be accessed in a subsequent read operation. If a user works with a map q25_f21.map say, and then finds it was a F(o)-F(c) map by mistake, he might be tempted to use the same file name after correcting the mistake. In such a case it would be necessary to clear the map cache before using the new map.