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  What is an optimizable expression?:

  An optimizable expression is any logical expression the takes on one of
  the following forms.

       <   index key    > < relational operator > < constant value >

  An < index key > may be any key of the open indexes in the currently
  selected work area.  Valid relational operators are "=, ==, !=, #, <>, <,
  >, <=, >=, $".   The < constant value > may be any expression that
  evaluates to a constant value.  This includes literal values such as
  "100.00", or any memory variable, field, or user defined function.

  Multiple optimizable expressions may be joined with the .AND. and .OR.
  logical operators to form complex optimizable expressions.  It is not
  necessary for all expressions in the FOR clause be an index key.  Non-
  indexed expressions can still be optimized through partial optimization.