code: @Override
      public String getProxyURL() {
          try {
              if (hasProperty(PROXY_FOR)) {
                  return getProperty(PROXY_FOR).getString();
              }
              return null;
          } catch (final RepositoryException e) {
              throw new RepositoryRuntimeException(e);
          }
      }
docs: /*
  (non-Javadoc)
  @see org.fcrepo.kernel.api.models.FedoraBinary#getProxyURL()