Coding classes in Fermanagh and Omagh

Historians have Latin names for the two kinds of clue: a terminus post quem is the earliest a thing can be, and a terminus ante quem is the latest. In code both become a pair of numbers, earliest and latest. To combine two ranges you take the later of the two starts and the earlier of the two ends. If the start is still before the end, that overlap is the answer. The learner writes that one small function, feeds it the clues from the record one at a time, and prints how the window shrinks.

Two lessons come out of the table. First, some clues add nothing: the closing date of 1931 is true but does not narrow anything, and a good program can report which clues did the work. Second, the last row is different in kind. The evidence alone gives 1834 to 1842; saying the bridge dates from the same decade adds an expert's judgement that it was built during the canal works rather than at their very end. Code keeps the two apart, so a reader can see which part is proof and which is judgement.

Then the learner adds a made-up clue, a datestone reading 1829, and the overlap vanishes: the start comes after the end. The program must not just crash. It tries removing each clue in turn and finds two ways to restore agreement: drop the datestone, or drop the map. Code cannot say which to trust; that takes judgement about sources. There is also a subtler point: a map shows what was standing when it was surveyed, which can be earlier than the year printed on it. And the classic slip is using the earlier start and the later end, which takes the union rather than the overlap and widens the answer to 1825 to 1931.

Separating proof from judgement is a good reason for a Fermanagh and Omagh teenager to keep coding in 2026. The longer argument is in why teenagers should still learn to code in 2026.