code: @Override
  	public String getIdentifier() {
  		if (ranges.isEmpty()) return pdbId;
  		return pdbId + "." + ResidueRange.toString(ranges);
  	}
docs: Get the String form of this identifier.
  
  This provides the canonical form for a StructureIdentifier and has
  all the information needed to recreate a particular substructure.
  
  Example: 3iek.A_17-28,A_56-294
  @return The String form of this identifier