With SET FILTER:

  SET FILTER is now fully optimizable by Mach SIx.  Now filtering records
  on large database is as easy as SET FILTER TO.  It's really that easy!
  You can now eliminate all those complicated TBrowse() skip blocks,
  temporary indexes, and even scopes.  Yes even SCOPEs.  While scoping
  will continue to be a powerful filter tool in the SIx Driver, Mach SIx
  surpasses it's capability by allowing complicated filter conditions
  that utilize multiple indexes.

  Guidelines for an optimizable filter are the same as for the conditions
  specified in a query command.  To be optimizable, the filter must contain
  at least one active index key ( See m6_IsOptimize() ).  Remember, no
  LOCALs, STATICs, or extended expressions ( Clipper's macro compiler can't
  process them ) can be used in the filter condition.  For a discussion on
  how you can work around this limitation see With Locals/Statics in
  the USING section of this guide.

  Now you have two choices for optimizing commands with a query condition.
  First you can set a filter for a condition then issue the particular
  database command you want to optimize, or you can specify the query
  condition as part of the FOR clause of that command.  To make it even
  more complicated you could set a filter for one condition, then issue
  optimizable command on top of the filter with its own specific query
  condition.  It's all optimizable by Mach SIx.

  We've also added a function m6_IsFilter() so you can check the
  optimization level of the active filter condition.  REFRESH FILTER
  gives you the control to specify when the filter should updated on
  shared files thus eliminating the overhead of constantly maintaining
  the filter while others do updates.

  If you're like most, you abandoned SET FILTER long ago.  But now with
  the power of Mach SIx the possibilities are endless!