Advent of Code day 2

[Stephen Leake <stephen_leake@stephe-leake.org>]

This is an example of the customer problem spec not being complete; in
the example, all the letter positions are single digit, so my first
draft assumed that, which made finding the values trivial. But then the
real input has 1 or 2 digit numbers.

Handling that took me a while; first I had to actually understand the
semantics of Ada.Strings.Index (Source, Set, Test); "the smallest index
satisfying the test" does _not_ produce a span of characters matching
the test; I should have used Gnatcoll.regexp.

Then I found an indent bug in the development version of emacs ada-mode,
which I took a detour to fix. Which is one reason to do this contest;
writing Ada code is a good way to find bugs in ada-mode. (just out of
curiosity; what development environments are you all using?)

Then for the second puzzle, the fact that Ada String slices _don't_ have
first index 1 confused things; I ended up writing a Slide function to
keep the logic simpler.

Finally got it right.

I was hoping to use 'Reduce in the sled rental count, but GNAT Community
2020 -gnat2020 -gnatX doesn't support that. I did use @, which I like a
lot.

-- 
-- Stephe

Written from unceded ancestral homelands of the Huichiun, an Ohlone
people,for which I pay Shuumi Land Tax