// sync source
module.exports = function (options) {
  return function read (buffer|length?) {
    //if buffer is passed, it should be filled instead of created
    //if length is passed, a new buffer of that length should be created
    //return null to indicate end
    return buffer
  }
}
// async source
module.exports = function (options) {
  //pass null as a callback to indicate the end
  return function read (buffer|length?, cb) {
    //cb(null, null) to end
    //cb(error) to throw error
    cb(null, buffer)
  }
}

// sync through
module.exports = function (options) {
  return function transform (buffer) {
    //return null to end
    return buffer
  }
}
// async through
module.exports = function (options) {
  return function transform (buffer, cb) {
    //cb(null, null) to end
    //cb(error) to throw error
    cb(null, buffer)
  }
}

// sync sink
module.exports = function (options) {
  return function write (buffer) {
    // do stuff
    // throw error
    // if buffer is null - end writer
  }
}
// async sink
module.exports = function (options) {
  //do urgent end
  write.end = function () {}  
  return write;
  function write (buffer, cb) {
    // do stuff
    // if buffer is null - plan ending
    cb(err, data);
  }
}