SayHelp screen 8: Writing your own filter.   [Part two, extending sayfilter]
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The command 'sayfilter <ch> <function> <params>' is passed to filters using
the same entrypoint as say, and you should check for it using 'command @'.
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Minimally:     command @ "sayfilter" stringcmp not if 0 exit then
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It is provided to make setting filter properties easier for the user.
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You can choose your own function names, but don't use 'here', 'ooc', 'say',
 'def', or any 1 or 2 character sequences lest they are read as <ch>.
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The '.n' system (shown with the replace filter) is organised by say itself and
is transparent to filters. You will not get the '.n' on the function word.
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On entry, n, db and <ch> are appropriately set, message=='<function> <params>'.
Use return value 0 if you don't recognise <function>.
Use return 1 if you do, having set any properties you want to set in response.
 Return any errors in the me_notify string, or blank or '>> Done' if no errors.
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sayhelp 5:rooms/envs  6:puppets  7,8,9:filter-writing  10:@verb  11:boringadmin