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  Optimizable Expressions:

  An optimizable expression is any expression found within the FOR clause
  of a supported command/function that takes on the following form:

      < 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.  All component
  expressions in a FOR clause need not be index key expressions. A FOR clause
  will still benifit from partial optimization if it contains both indexed
  and non-indexed expressions.

  EXAMPLES:

  If the following tags and keys were active...

      INDEX or TAG          INDEX KEY
      ------------          ---------
      SALARY                Salary
      NAME                  Last+First
      DATE                  Date
      STATE                 State

  ...then the following would be fully optimizable expressions:

    .  Salary > 25000 .AND. Salary < 30000

    .  Last = "Hewson"

    .  Salary > 25000 .AND. Last = "A" .AND. State = "CA"

  Expressions that contain at least one indexed expression are still
  partially optimized.  Note the non-indexed fields.

    .  State = "FL" .AND. Age = 21

    .  Salary > 25000 .AND. Last = "A" .AND. State = "CA" .AND. Zip = "92656"

  If an expression does not contain any optimizable components, the operation
  will simply default to what Clipper would have done in the first place!

  The following are the instances when optimization will not take place.

     . No database in use.
     . Not a SIx Driver workarea.
     . No index(es) in use.
     . Database contains over 524 million records.
     . No matching index keys found.
     . Not enough memory.
     . FILTER/FOR condition contains LOCAL or STATIC variables.
     . Not a logical condition.
     . FILTER/FOR condition contains an extended expression.
     . Matching key found, but in RYO, UNIQUE, or CONDITIONAL
           index that the order is not currently set to.
     . Mach SIx was not linked in (properly).