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   It is an expression that performs a validation check on the value of the
   variable. The expression inserted must return a value of .T. (True) or .F.
   (False).

   The Data not valid if parameter in the Options allows to verify the
   data entered by the operator in the field. The expression set must return
   a logical value, i.e. true (.T.) or false (.F.).

   During insertion of the expression, the Alt-1 keys allow to paste
   names of files, fields, and user defined functions from the Data
   Dictionary. If positioned on the list of user functions, the Ins key
   allows to add a new function on-the-fly.

Example:

   The data entered in the Supplier Discount (SupDsct) fields must be
   comprised between 5 and 15 percent. The expression entered will thus be:

     (SupDsct<5 .OR. SupDsct>15)