code: public int getLength(ResidueNumber start, ResidueNumber end) {
  		if( ! start.getChainName().equals(end.getChainName())) {
  			throw new IllegalArgumentException(String.format(
  					"Chains differ between %s and %s. Unable to calculate length.",
  					start,end));
  		}
  		Integer startPos = getPosition(start);
  		Integer endPos = getPosition(end);
  		if(startPos == null) {
  			throw new IllegalArgumentException("Residue "+start+" was not found.");
  		}
  		if(endPos == null) {
  			throw new IllegalArgumentException("Residue "+start+" was not found.");
  		}
  		return getLength(startPos, endPos, start.getChainName());
  	}
docs: Calculates the number of atoms between two ResidueNumbers, inclusive. Both residues
  must belong to the same chain.
  @param start First residue
  @param end Last residue
  @return The number of atoms from A to B inclusive
  @throws IllegalArgumentException if start and end are on different chains,
  or if either of the residues doesn't exist