Rename interval_{lo,hi} to result_{lo,hi}
Protect binop assertions (only valid when CHECK_BI); remove uniform invariants (commented out anyway)
Strict inequality for rhs of pair interval

Given (a,b) we used to interpret this as sum_{a <= i <= b} len[i]
But this has a problem when we need an identity element.
So, now we interpret (a,b) as sum_{a <= i < b} len[i].
Pair downsweep invariant and change representation to uint2