docs: function getPushClient(settings) {
      var sender = null;
  
      return function push(message, options, callback) {
        if (!message) {
          return callback(new Error("Missing required 'message' parameter"));
        }
        if (!options) {
          return callback(new Error("Missing required 'options' parameter"));
        }
        if (!options.broadcast) {
          if (!options.apps) {
            return callback(new Error("Missing required 'options.apps' parameter while 'options.broadcast' not specified" + JSON.stringify(options)));
          }
        }
  
        if (!sender) {
          sender = require('unifiedpush-node-sender').Sender(settings);
        }
  
        sender.send(message, options, callback);
      };
    }