wip: narrowing B2
need to be able to diff structured addresses (tuples)

using the regular + and * -1 operators, which is a bit of a hack
we should have a proper typed diff at some point
detect different tuples early
precedence
preliminary more general range splitting
inject lower and upper bound info

fixindex/trip count now computed during interation
n<17 test fails
testB2 works. yay!