code: function redisCheckCertificate(options, callback) {
      _debug('le-store-redis.redisCheckCertificate:',
        options.domains, options.email, options.accountId);
  
      if(options.domains && options.domains.length > 0) {
        return _getByIndex('idx-d2c', options.domains[0], callback);
      } else if(options.email) {
        return _getByIndex('idx-e2c', options.email, callback);
      } else if(options.accountId) {
        return _getByIndex('idx-a2c', options.accoundId, callback);
      }
  
      callback(new Error('le-store-redis.redisCheckCertificate requires ' +
        'options.domains, options.email, or options.accoundId'));
    }
docs: Checks to see if a certificate exists in the database. The provided options
  describe how the certificate should be looked up.
  
  @param {Object[]} options - options passed to check call.
  @param {string} options[].domains - domains to use when looking
  up the account. These will be used for the lookup first.
  @param {string} options[].email - optional email address to use when
  looking up the certificate.
  @param {string} options[].accountId - optional accountId to use when
  looking up the certificate.
  @param {Function} callback(err, cert) - called when an error occurs, or
  when a certificate is successfully retrieved from the database.