package sql_boolean_pkg is 
   type boolean_with_unknown is (false, unknown, true); 
   function "not" (left : boolean_with_unknown) return boolean_with_unknown;
   
   function "and" (left, right : boolean_with_unknown) 
      return boolean_with_unknown;
   
   function "or" (left, right : boolean_with_unknown) 
      return boolean_with_unknown;
   
   function "xor" (left, right : boolean_with_unknown) 
      return boolean_with_unknown;
   
   function to_boolean (left : boolean_with_unknown) return boolean;
   
   function is_true (left : boolean_with_unknown) return boolean;
   
   function is_false (left : boolean_with_unknown) return boolean;
   
   function is_unknown (left : boolean_with_unknown) return boolean;
   
end sql_boolean_pkg;