code: function(certs, index)
    {
      var cert = certs[index];
      validateCert(cert, function(err, data)
      {
        completed++;
        errors[index] = err;
  
        // if we have an ocsp response, cache it
        if (data)
        {
          getOcspResponseCache().set(cert, data);
        }
  
        // if this is the last request to complete
        if (completed === certs.length)
        {
          // if we saw one or more errors, invoke the callback with the first
          // error we saw; otherwise invoke the callback without any error
          for (var errorIndex = 0, length = errors.length;
               errorIndex < length; errorIndex++)
          {
            var error = errors[errorIndex];
            if (error)
            {
              break;
            }
          }
          cb(error);
        }
      });
    }
docs: Called for every certificate as we traverse the certificate chain and
  validate each one.
  
  @param certs
  @param index