@Documented
 @Retention(value=RUNTIME)
 @Target(value=TYPE)
public static @interface DoFn.UnboundedPerElement
DoFn
 specifying that the DoFn performs an unbounded amount of work per input element, so
 applying it to a bounded PCollection will produce an unbounded PCollection. It
 is an error to specify this on a non-splittable DoFn.