repeated nested access

[mtoy-googly-moogly]

for something like this, which we see a lot in real nested data ...

aggregate: avg_calc is avg(longJoinName.nestedData.containsXandY.x - longJoinName.nestedData.containsXandY.y)

Something like the pascal with statement ...

aggregate: avc_calc is with longJoinName.nestedData.containsXandY { avg(x - y) }