fn select_word(
        &mut self,
        condition: impl Into<Self::Expr> + Clone,
        a: Word<impl Into<Self::Expr> + Clone>,
        b: Word<impl Into<Self::Expr> + Clone>,
    ) -> Word<Self::Expr> {
        Word(array::from_fn(|i| self.if_else(condition.clone(), a[i].clone(), b[i].clone())))
    }
Same as `if_else` above, but arguments are `Word` instead of individual expressions.
crates/core/machine/src/air/word.rs