docs: function checkOpts(config, cb) {
    // jshint eqnull: true
  
    var prefix = "ragents.createSession(): "
  
    if (config == null) throw Error(prefix + "config is null")
  
    if (config.url == null) throw Error(prefix + "config.url is null")
    if (config.key == null) throw Error(prefix + "config.key is null")
  
    if (null == cb)        throw Error(prefix + "callback is null")
    if (!_.isFunction(cb)) throw Error(prefix + "callback is not a function")
  }