Function is_non_ascii_letter 

pub fn is_non_ascii_letter(c: char) -> bool

Whether a character would be an identifier character in some language but is not one in Beck.

Used only to turn “unrecognised character” into a diagnostic that says why: a Cyrillic а in an identifier is not a typo, it is either a mistake worth naming or an attack, and either way “not a Beck token” is the least useful thing to say about it.