Function unconstrained_bit_selectors 

pub fn unconstrained_bit_selectors<'a, F: TowerField>(
    specs: &'a [(String, PermutationCheckSpec)],
    ast: &ConstraintAst<F>,
    virtual_layout: &[ColumnType],
) -> Vec<(usize, &'a str)>

Non-paired Bit selectors with no direct s·s + s boolean root, each tagged with the declaring bus_id. Advisory only: booleanness can hold indirectly (one-hot, disjoint products), callers warn rather than reject.