code: @Override
      public void copyFrom(Frame<IsNullValue> other) {
          super.copyFrom(other);
          decision = ((IsNullValueFrame) other).decision;
          if (trackValueNumbers) {
              knownValueMap = Util.makeSmallHashMap(((IsNullValueFrame) other).knownValueMap);
          }
      }
docs: /*
  (non-Javadoc)
  
  @see edu.umd.cs.findbugs.ba.Frame#copyFrom(edu.umd.cs.findbugs.ba.Frame)