Hm, I see.
For one, AIR doesn't use XML (though you could write an xml/AIR converter) -- but the underlying idea (having an exposed, manipulable AST) is essentially the same, going by this http://sireum.github.io/doc/bakar/
OCTAGRAM: Also, it looks like the gnat2xml is based on ASIS, which has subtly differences from DIANA (upon which AIR is based) -- see: http://archive.adaic.com/work-grp/asiswg/asis/v2.0/asis_cd3.txt (D.2.2)
OCTAGRAM: Also, ASIS is "an interface between an Ada environment and any tool or application requiring information from it" -- as per ASIS FAQ: http://www.sigada.org/WG/asiswg/ASIS_FAQ.html -- which is different than being an intermediate representation between ends [and possibly tools] that DIANA was.
OCTAGRAM: Welcome back.