code: public IUnitizingAnnotationUnit addUnit(long offset, long length,
  			int raterIdx, final Object category) {
  		IUnitizingAnnotationUnit unit = createUnit(offset, length,
  				raterIdx, category);
  		units.add(unit);
  		return unit;
  	}
docs: Creates a new {@link UnitizingAnnotationUnit} and adds it to the
  present annotation study. The parameters define the position
  of the unit within the continuum (indicated by the offset/start position
  and the length of the unit), the rater who identified this unit and
  the category assigned to it. If the unitzing study does not
  distinguish any categories, use some dummy object, such as "X"
  to distinguish identified units from gaps (i.e., units whose
  category is null).