Two kinds of slot, no default

Every slot is either fix (can’t rebind) or mut (can) — always stated, no default to memorize.

That’s deliberate. let is immutable in Rust but mutable in JavaScript; const, val, and var all disagree depending who you ask. fix/mut don’t borrow anyone’s baggage — they just read as what they do.

mut score = 0;
score = score + 10;
score

A mut slot rebinds freely. A fix slot doesn’t:

fix score = 0;
score = score + 10;
score

Fix that error yourself — you’ve got two honest options: make the slot mut, or stop reassigning it and compute the new value under its own name.

(This is only about rebinding the name. Whether a value can change in place at all is a different question — next screen.)