Also people at Sabre (worked there) and I presume Amadeus (those are two main players - surprised you seemed to "forget" to mention them?).
As to the "categories", people at Sabre working with them used to say in some cases you can not even be sure if the calculation (of a ticket price - "fare") will end in finite time, so the "macro language" is apparently Turing-complete!
I wasn't meaning to slight either Amadeus or Sabre, and indeed they have most of the other half of the people who understand this stuff between them. The others are at various airlines around the world and at ATPCO.
I've written elsewhere that I personally think the original Sabre system was/is one of the most impressive accomplishments in the history of computing. Using modern tools made things easier for us at ITA, though we compensated for that by trying to compute the entire (very large) solution space for every query, where prior systems used heuristics.
As to the "categories", people at Sabre working with them used to say in some cases you can not even be sure if the calculation (of a ticket price - "fare") will end in finite time, so the "macro language" is apparently Turing-complete!