"Mirror worlds" is only one of David Gelernter's big ideas. Another is "lifestreams"—in es

sence, vast electronic diaries. "Every document you create and every document other people send you are stored in your lifestream," he wrote in the mid-1990s together with Eric Freeman, who produced a doctoral thesis on the topic. Putting electronic documents in chronological order, they said, would make it easier for people to manage all their digital output and experiences.

Lifestreams have not yet replaced the desktop on personal computers, as Mr. Gelernter had hoped. Indeed, a software start-up to implement the idea folded in 2004. But today something quite similar can be found all over the web in many different forms. Blogs are essentially electronic diaries. Personal newsfeeds are at the heart of Facebook and other social networks. A torrent of short text messages appears on Twitter.

Certain individuals are going even further than Mr. Gelernter expected. Some are digitising their entire offices, including pictures, bills and correspondence. Others record their whole life. Gordon Bell, a researcher at Microsoft, puts everything he has accumulated, written, photographed and presented in his "local cyberspace". Yet others "log" every aspect of their lives with wearable cameras.

The latest trend is "life-tracking". Practitioners keep meticulous digital records of things they do: how much coffee they drink, how much work they do each day, what books they are reading, and so on. Much of this is done manually by putting the data into a PC or, increasingly, a smartphone. But people are also using sensors, mainly to keep track of their vital, signs, for instance to see how well they sleep or how fast they run.

The first self-trackers were mostly liber-geeks fascinated by numbers. But the more recent converts simply want to learn more about themselves, says Gary Wolf, a technology writer and co-founder of a blog called "The Quantified Self". They want to use technology to help them identify factors that make them depressed, keep them from sleeping or affect their cognitive performance. One self-tracker learned, for instance, that eating a lot of butter allowed him to solve arithmetic problems faster.

A market for self-tracking devices is already emerging. Fitbit and Greengoose, two start-ups, are selling wireless accelerometers that can track a user's physical activity. Zeo, another start-up, has developed an alarm clock that comes with a headband to measure people's brainwave activity at night and chart their sleep on the web.

As people create more such self-tracking data, firms will start to mine them and offer services based on the result. Xobni, for example, analyses people's inboxes ("xobni" spelled backwards) to help them manage their e-mail and contacts. It lists them according to the intensity of the electronic relationship rather than in alphabetical order. Users are sometimes surprised by the results, says Jeff Bonforte, the firm's boss: "They think it's creepy when we list other people before their girlfriend or wife. "

According to the passage, "lifestream" are______.

A．documents about an individual daily life

B．software storing electronic diaries

C．streams in one's life

D．a great idea about putting electronic documents in time order