The book works through what happens when tensor coordinates have names the compiler can check, from reductions through autodiff and recurrences. There's a small prototype language (github.com/einlang/einlang), but the book is the main thing.
Actually this library (match(it)) is more similar to Racket. You can see there are lots of patterns borrowed from Racket pattern matching, say app pattern and ooo pattern.
Sadly lvariant did not get into C++ language. The library supports pattern matching against std::variant/std::any and class inheritance as a replacement.