code: public ResidueNumber getResidue(int positionInRange, AtomPositionMap map) {
  		if (map == null) throw new NullPointerException("The AtomPositionMap must be non-null");
  		int i = 0;
  		for (Map.Entry<ResidueNumber, Integer> entry : map.getNavMap().entrySet()) {
  			if (i == positionInRange) return entry.getKey();
  			if (contains(entry.getKey(), map)) {
  				i++;
  			}
  		}
  		return null;
  	}
docs: Returns the ResidueNumber that is at position {@code positionInRange} in
  <em>this</em> ResidueRange.
  @return The ResidueNumber, or false if it does not exist or is not within this ResidueRange