code: function Prefetch(o) {
    this.url = o.url;
    this.ttl = o.ttl;
    this.cache = o.cache;
    this.prepare = o.prepare;
    this.transform = o.transform;
    this.transport = o.transport;
    this.thumbprint = o.thumbprint;
  
    this.storage = new PersistentStorage(o.cacheKey);
  }
docs: constructor ----------- defaults for options are handled in options_parser