Dbf:ChValid(bValid): public: return true
  To last created object of OneDbf class is for last defined database field
  stored code block Valid . This code block is processed after finishig of
  editation of this database field only when the database filed was changed.
  If returning false the field editation  can't be finished. (incorrect
  value entered). The code block "bValid", when
  evaluated, will be passed this parameters:
   - currently edited (clipper) Get object.
   - logical value CanEdit (true if editting allowed)
   - old edited value, (it is saved into temporary variable before editting
     of the Get object)