Link the old context-sensitive proof.
diff --git a/src/tokens.md b/src/tokens.md
index 613f66a..5437800 100644
--- a/src/tokens.md
+++ b/src/tokens.md
@@ -216,6 +216,7 @@
as a destructive operation which forces you to forget the count. This allows
for the "comparison" of two counts (such (as (balanced) (parens))) but not 3+.
+See [the proof for more formal details](https://github.com/rust-lang/rust/blob/5187be620c76a313a19b9b596e1bce3a80a345dd/src/grammar/raw-string-literal-ambiguity.md).
Examples for string literals:
```rust
