code: function parse(input, fn) {
    let returnValue;
    const transformed = transformPair(input, (readable, writable) => {
      const reader = getReader(readable);
      reader.remainder = () => {
        reader.releaseLock();
        pipe(readable, writable);
        return transformed;
      };
      returnValue = fn(reader);
    });
    return returnValue;
  }
docs: Parse a stream using a helper function which is passed a Reader.
  The reader additionally has a remainder() method which returns a
  stream pointing to the remainder of input, and is linked to input
  for cancelation.
  @param {ReadableStream|Uint8array|String} input
  @param {Function} fn
  @returns {Any} the return value of fn()