SayHelp screen 8: Writing your own filter.   [Part two, extending sayfilter]
The command 'sayfilter <ch> <function> <params>' is passed to filters using
the same entrypoint as say, and you should check for it using 'command @'.
Minimally:     command @ "sayfilter" stringcmp not if 0 exit then
It is provided to make setting filter properties easier for the user.
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>.
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.
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.
sayhelp 5:rooms/envs  6:puppets  7,8,9:filter-writing  10:@verb  11:boringadmin