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Apple engineers have long had a delightfully twisted sense of humour. Way back in the early days of Macs, number crunching used a wonderful library of functions known as the Standard Apple Numerics Environment, or Sane, when programming with it was often anything but. One of Sane's features was that calculations could return a value that was 'not a number', a NaN, and you may still stumble across software that, when you divide a number by zero, displays the result as NaN rather than Excel's row of hashes. In some cases, particularly in those early days, the outcome of certain calculations could be 'result not defined', although no one tried to form that into the inevitable acronym of Rand.

Over the years, fewer results in computing have remained not defined. However, the otherwise welcome trend away from blockbuster printed manuals has started to reverse this. Despite thousands of pages of freely downloadable documentation for Mac OS X and Mac OS X Server, there are a surprising number of features that are only minimally described, leaving us to stumble around in the dark.

On the other hand, in most matters computing, we can always experiment and find out for ourselves. This is quite different from medicine or physiology, where you can't even try a quick lunchtime 'what if' on yourself any more. Pioneers such as Werner Forssmann, who performed the first cardiac catheterisation on himself in 1929, would now be sacked for breaching health and safety regulations,

Sometimes such self-experiments in Mac use are rather more committing. Assuming you, too, have been enjoying Time Machine's elegantly simple backups, what did you expect to happen as your backup disk started to run out of space? Among the rumours and reports I've seen were: older backups will be successively merge-deleted to accommodate newer ones; older backups will be compressed to free up space; older backups will just be trashed; Time Machine will crash; and Time Machine will simply stop making backups to that volume.

Initial searches of the scant documentation proved unhelpful, and this smacked of being a 'result not defined' - hardly a good answer for something as vital as a backup system. I was increasingly frustrated by Apple's inconsistent approach to electronic documentation, too. Mac OS X Server has the most extensive suite, although barren of illustrations. The Xcode developer toolkit has even more, and in many cases is the best place to look for answers to technical questions, but Time Machine isn't intended for third-party development. Apart from the rather fragmentary Apple Help, hardly conducive to in-depth study, using Leopard seems quite poorly described in official material.

In fact, Time Machine does the simplest and most conservative thing: it keeps making backups according to its hourly, daily and weekly regime until your backup volume is almost full, then stops and leaves you to find it another backup volume. This is fine by me, as I will soon pop out that hard disk to keep as an archive and replace it with a blank drive for when it runs out of space on its current and second disk. However, that shouldn't be the only option, and I wonder whether during Leopard's lifetime we will be able to customise this behaviour. I would like to be able to generate a full backup as at two months ago, discarding previous changes.

Let's hope Apple's technical authors reach Time Machine in the near future, so that its results will no longer be not defined.