public Instant watermarkHold(){
  long min=Long.MAX_VALUE;
  for (  Instant hold : watermarkHolds.values()) {
    min=Math.min(min,hold.getMillis());
  }
  return new Instant(min);
}

public Instant watermarkHold ( ) { long min = Long . MAX_VALUE ; for ( Instant hold : watermarkHolds . values ( ) ) { min = Math . min ( min , hold . getMillis ( ) ) ; } return new Instant ( min ) ; }
returns the minimum over all watermark holds .
