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Click, hum, click, hum, click, hum. Click, click, click, click, click, hum. Hmmm. A low-level supervising program woke up a slightly higher-level supervising program deep in the ship’s semisomnolent cyberbrain and reported to it that whenever it went click all it got was a hum. The higher-level supervising program asked it what it was supposed to get, and the low-level supervising program said that it couldn’t remember what it was meant to get, exactly, but thought it was probably more of a sort of distant satisfied sigh, wasn’t it? It didn’t know what this hum was. Click, hum, click, hum. That was all it was getting. The higher-level supervising program considered this and didn’t like it. It asked the low-level supervising program what exactly it was supervising and the low-level supervising program said it couldn’t remember that either, just that it was something that was meant to go click, sigh every ten years or so, which usually happened without fail. It had tried to consult its error look-up table but couldn’t find it, which was why it had alerted the higher-level supervising program of the problem. The higher-level supervising program went to consult one of its own look-up tables to find out what the low-level supervising program was meant to be supervising. It couldn’t find the look-up table. Odd. It looked again. All it got was an error message. It tried to look up the error message in its error message look-up table and couldn’t find that either. It allowed a couple of nanoseconds to go by while it went through all this again. Then it woke up its sector function supervisor. The sector function supervisor hit immediate problems. It called its supervising agent, which hit problems too. Within a few millionths of a second virtual circuits that had lain dormant, some for years, some for centuries, were flaring into life throughout the ship. Something, somewhere, had gone terribly wrong, but none of the supervising programs could tell what it was. At every level, vital instructions were missing, and the instructions about what to do in the event of discovering that vital instructions were missing, were also missing. Small modules of software—agents—surged through the logical pathways, grouping, consulting, regrouping. They quickly established that the ship’s memory, all the way back to its central mission module, was in tatters. No amount of interrogation could determine what it was that had happened. Even the central mission module itself seemed to be damaged. This made the whole problem very simple to deal with, in fact. Replace the central mission module. There was another one, a backup, an exact duplicate of the original. It had to be physically replaced because, for safety reasons, there was no link whatsoever between the original and its backup. Once the central mission module was replaced it could itself supervise the reconstruction of the rest of the system in every detail, and all would be well.
”
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Douglas Adams (The Ultimate Hitchhiker's Guide to the Galaxy (Hitchhiker's Guide to the Galaxy #1-5))