Lean 4 fork for HoTT-compatible kernel extensions (Path types, transport, HITs). Maintained against upstream leanprover/lean4.
d6cd738ab4
This PR introduces safe alternatives to `String.Pos` and `Substring` that can only represent valid positions/slices. Specifically, the PR - introduces the predicate `String.Pos.IsValid`; - proves several nontrivial equivalent conditions for `String.Pos.IsValid`; - introduces `String.ValidPos`, which is a `String.Pos` with an `IsValid` proof; - introduces `String.Slice`, which is like `Substring` but made from `String.ValidPos` instead of `Pos`; - introduces `String.Pos.IsValidForSlice`, which is like `String.Pos.IsValid` but for slices; - introduces `String.Slice.Pos`, which is like `String.ValidPos` but for slices; - introduces various functions for converting between the two types of positions. The API added in this PR is not complete. It will be expanded in future PRs with addional operations and verification. |
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