block_filter

      command: <string>

 The marked block is written to temporary file 1. The command is executed
 with temporary file 1 as its standard input. The standard output is
 collected in temporary file 2. The marked block is deleted and replaced by
 the contents of temporary file 2. Both temporary files are then deleted.

 The reserved comments are written using those appropriate for the current
 buffer.

 eg:  block_filter "grep the"

 The commands are executed via the "C" system() call. Therefore COMMAND.COM
 or some other command processor should be accessible by the COMSPEC
 environment variable on a DOS system. On OS/2 CMD.EXE should be
 accessible.

 If AE is executed with the -r option, it is running in 'restricted' mode,
 and so will not shell out.

 NOTE: Fold information is also passed through the filter command.
       Therefore if you feed such data through a command like SORT the
       resulting data may not form a correctly folded file.

       eg: The argument fold   File before SORT         File after SORT

           {{{ this fold       /*...sthis fold:0:*/     contents
               contents        contents                 /*...e*/
           }}}                 /*...e*/                 /*...sthis fold:0:*/

           Note that the resulting file has its end reserved comment before
           its start! If you passed the above file through an
           "uppercaseifier" then you would find that the reserved comments
           would be "uppercaseified" as well. In the reserved comment ...s
           implies closed fold, ...S would imply open. Therfore such a
           command would, as a side effect, open all nested folds!

           Moral: only use block_filter on text, unless you know the program
           knows about folding.

 NOTE: On UNIX and other environments, the commands you can give will vary
       and indeed on some machines, this feature may not be available at all.