code: public Observable<Page<CertificateItem>> getCertificateVersionsNextAsync(final String nextPageLink) {
          return getCertificateVersionsNextWithServiceResponseAsync(nextPageLink)
              .map(new Func1<ServiceResponse<Page<CertificateItem>>, Page<CertificateItem>>() {
                  @Override
                  public Page<CertificateItem> call(ServiceResponse<Page<CertificateItem>> response) {
                      return response.body();
                  }
              });
      }
docs: List the versions of a certificate.
  The GetCertificateVersions operation returns the versions of a certificate in the specified key vault. This operation requires the certificates/list permission.
  
  @param nextPageLink The NextLink from the previous successful call to List operation.
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the observable to the PagedList&lt;CertificateItem&gt; object