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kotlinlang

What characters valid Kotlin identifiers? Some unicode characters are allowed without backtick-escaping. For example `val ε = 0.03` is a valid statement, but `val ∇ = 0.03` is not.

Apparently it allows symbols from Unicode character categories `Lu`, `Ll`, `Lt`, `Lm`, `Lo`, `Nl` and `Nd`. The lexical grammar is defined here: <https://github.com/JetBrains/kotlin-spec/blob/master/kotlin-spec.asc#42-lexical-grammar>

You also can use backtickes:
```val `∇` = 0.03```

Yeah, but are also some restrictions on characters you can put inside backticks