 VARYING identifier-2 FROM { literal-2    } [ BY { literal-3    } ]
 ~~~~~~~              ~~~~ { identifier-3 }   ~~ { identifier-4 }
         [ UNTIL conditional-expression-1 ]
           ~~~~~
 [ AFTER identifier-5 FROM { literal-4    } [ BY { literal-5    } ]
   ~~~~~              ~~~~ { identifier-6 }   ~~ { identifier-7 }
         [ UNTIL conditional-expression-2 ] ]...
           ~~~~~