The user defined filter facility provides a mechanism whereby
   filters written as DEC Text Processing Utility (DECTPU) programs are
   automatically executed when tests are run. These filters are
   referred to as user filters. The implementation enables users to
   solve easily many filtering problems, often with a single-line
   program, whilst allowing full access to the facilities of DECTPU to
   solve more complex cases.

   To implement a new filter, a file containing the required DECTPU
   commands is created. There are a number of predefined patterns and a
   global replace procedure provided which can be used to build the
   commands. For example, the following command will replace device
   names that precede a directory with the string "DEVICE":

     global_replace( identifier + ':[' , 'DEVICE:[' )

   The filter can be tested using the /USER_FILTER qualifier to the
   FILTER command. For example:

     FILTER/USER_FILTER=DEVICE.TPU TEST.DAT

   See help on FILTER /USER_FILTER for more information.

   Alternatively, filters can be developed using the TPU
   pattern style feature of Language-Sensitive Editor.

   To associate a user filter with a test, a logical variable starting
   with the characters "DTM$UF_" is created. The value of the variable
   is the file specification of the file containing the DECTPU
   commands. For example:

     CREATE VARIABLE /LOGICAL DTM$UF_DEVICE "DISK1:[TEST.FILTERS]DEVICE.TPU"

   The variable is then associated with the test in the usual way, for
   example:

     MODIFY TEST /VARIABLE=DTM$UF_DEVICE

   When the test is run, as part of a collection, the filter will be
   applied.