code: public Pairtree getPrefixedPairtree(final String aPrefix, final File aDirectory) throws PairtreeException {
          return new FsPairtree(aPrefix, myVertx, getDirPath(aDirectory));
      }
docs: Gets a file system based Pairtree using the supplied directory as the Pairtree root and the supplied prefix as
  the Pairtree prefix.
  
  @param aPrefix A Pairtree prefix
  @param aDirectory A directory to use for the Pairtree root
  @return A Pairtree root
  @throws PairtreeException If there is trouble creating the Pairtree