Rework elaboration #136
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Rework elaboration #136
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This PR reworks substantial parts of the EL-to-IL elaborator. Most importantly, it replaces the previous ad-hoc uses of backtracking with a more systematic approach that allows more programs to type-check as expected. Error messages now provide information about which alternatives have been tried.
In IL and AL, the PR adds an expression form
LiftE e, which lifts at?to at*.The PR also introduces a new EL expression form
[e]for expressing lists. It is equivalent to a singletoneimplicitly injected into list type, but with more restrictive type checking. It can be use to disambiguate multi-nested expressions in a way that was previously impossible. For example, a[i*]* : nat**can now be distinguished from[i]** : nat**.I'd like to do further improvements and clean-ups as follow-ups, but this PR should be useful already.