code: function promisify_read_stream() {
      var transformer = function (stream_promise) {
          var res = new PStream();
          res.to = function (dest) {
              when(stream_promise, function (stream) {
                  PStream.wrap_read_stream(stream).to(dest);
              }, function (err) {
                  dest.error(err);
              });
          };
          return res.sanitize();
      };
  
      transformer.for_property = function (obj_promise, prop) {
          return transformer(when(obj_promise, function (obj) {
              return obj[prop];
          }));
      };
  
      return transformer;
  }
docs: Takes a promised read stream and returns a PStream