Function looker_denominator 

pub fn looker_denominator<A, F, P>(
    alloc: &A,
    c: F,
    index: &[usize],
) -> FieldVec<P, A>
where
    A: Allocator,
    F: BinaryField<Underlier: Divisible<u64>>,
    P: PackedField<Scalar = F>,

Build the looker denominator c - I over the n-variable looker cube.

Entry i is c - iota(index[i]), the logUp denominator for looker row i.

Preconditions

• index.len() is a power of two.