we’ve installed the foundation piles and are ready to start building Mirror worlds. In this chapter we discuss (so to speak) the basement, in the next chapter we get to the attic, and the chapter after that fills in the middle region and glues the whole thing together. The basement we are about to describe is filled with lots of a certain kind of ensemble program. This kind of program, called a Trellis, makes the connection between external data and internal mirror-reality. The Trellis is, accordingly, a key player in the Mirror world cast. It’s also a good example of ensemble programming in general, and, I’ll argue, a highly significant gadget in itself. The hulking problem with which the Trellis does battle on the Mirror world’s behalf is a problem that the real world, too, will be confronting directly and in person very soon. Floods of data are pounding down all around us in torrents. How will we cope? what will we do with all this stuff? when the encroaching electronification of the world pushes the downpour rate higher by a thousand or a million times or more, what will we do then? Concretely: I’m talking about realtime data processing. The subject in this chapter is fresh data straight from the sensor. we’d like to analyze this fresh data in “realtime”—to achieve some understanding of data values as they emerge. Raw data pours into a Mirror world and gets refined by a data distillery in the basement. The processed, refined, one-hundredpercent pure stuff gets stored upstairs in the attic, where it ferments slowly into history. (In the next chapter we move upstairs.) Trellis programs are the topic here: how they are put together, how they work. But there’s an initial question that’s too important to ignore. we need to take a brief trip outside into the deluge, to establish what this stuff is and where it’s coming from. Data-gathering instruments are generally electronic. They are sensors in the field, dedicated to the non-stop, automatic gathering of measurements; or they are full-blown infomachines, waiting for people to sit down, log on and enter data by hand.