code: function expand_r(options, lines, prefix) {
    Object.getOwnPropertyNames(options).forEach(function(key) {
      var full = prefix.concat(key);
      var value = options[key];
  
      if (Array.isArray(value)) {
        value.forEach(function(val) {
          lines.push(full.concat(val).join(' '));
        });      
      }
      else if (typeof(value) == 'object') {
        expand_r(value, lines, full);
      }
      else {
        lines.push(full.concat(value).join(' '));
      }
    });
  }
docs: Recursively expand `options` into `lines` with a `prefix`.
  
  @private
  @static
  @category udhcpd
  @param {object} options The dhcp server configuration.
  @param {array} lines The lines of the configuration file.
  @param {array) prefix The key prefix.