code: public static PropertyFile readOnlyUnion(final File... filenames) throws IOException
  	{
  		final PropertyFile props = new PropertyFile();
  
  		for (final File f : filenames)
  		{
  			if (f != null && f.exists())
  			{
  				props.merge(new PropertyFile(f));
  			}
  		}
  
  		props.readOnly = true;
  
  		return props;
  	}
docs: Creates a read-only union of a number of property files<br />
  If any property file is null or the file it points to does not exist then it is ignored
  
  @param filenames
  
  @return
  
  @throws IOException
  if an unexpected error occurs while loading a file