The vitae knows where it is at all times. It knows this because it knows where it isn't. By subtracting where it is from where it isn't, or where it isn't from where it is (whichever is greater), it obtains a difference, or deviation. The arm subsystem uses deviations to generate corrective instructions to drive the vitae from a hex where it is to a hex where it isn't, and arriving at a hex where it wasn't, it now is. Consequently, the hex where it is, is now the hex that it wasn't, and it follows that the hex that it was, is now the hex that it isn't.
In the event that the hex that it is in is not the hex that it wasn't, the solution has acquired a variation, the variation being the difference between where the vitae is, and where it wasn't. If variation is considered to be a significant factor, it too may be corrected by the Glyph of Disposal. However, the vitae must also know where it was.
The vitae guidance program scenario works as follows. Because a variation has modified some of the information the vitae has obtained, it is not sure just where it is. However, it is sure where it isn't, within reason, and it knows where it was. It now subtracts where it should be from where it wasn't, or vice-versa, and by differentiating this from the algebraic sum of where it shouldn't be, and where it was, it is able to obtain the deviation and its variation, which is called meow.