code: public static int calcLength(List<ResidueRangeAndLength> rrs) {
  		int l = 0;
  		for (ResidueRangeAndLength rr : rrs) {
  			l += rr.getLength();
  		}
  		return l;
  	}
docs: Calculates the combined number of residues of the ResidueRanges in {@code rrs}.
  
  Assumes no overlap. If two or more ranges cover the same residues, will over-count the union of the residues.
  
  @param rrs
  A list of ResidueRanges
  @return The combined length
  @throws IllegalArgumentException
  If the {@link #getLength() length} of one or more ResidueRange is null
  @see #getLength()