public Sentence(Cell[] units, int offset) {
		if (units == null)
			throw new IllegalArgumentException("units must be not null");
		if (units.length < 3 || !units[0].pos.equals(Tag.ST)
				|| !units[units.length - 1].pos.equals(Tag.SE)) {
			throw new IllegalArgumentException(
					"units must be not empty and must begin with ST and end with SE");
		}

		this.units = units;
		this.offset = offset;
	}