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Previously, when indexing into a sequence, there were code paths in both the SBV and What4 backends where an if/then/else tree would be built to compute the value of a symbolic lookup, with one branch for each valid sequence index. They differed, however, in what value they would comute for out-of-bound indices. For SBV, we would compute a `zero` value, whereas for What4 we would raise an error. For What4, this resulted in unfortunate and redundant proof obligations, as we elsewhere add a test for in-bounds contditions to the safety predicate. Moreover, the strategy of returning a `zero` value could panic in some situations, as not every type has a `zero` value anymore (e.g., newtypes). In both cases, we now do the same thing, which is not to have a case of the if/then/else tree for out-of-bounds indices; the final element of the sequence will simply be returned instead. We will only raise an out of bounds error in this code path if the sequence is actually empty. Fixes #1239
This tracks changes to the symbolic indexing primitives.
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