Function combine_table_entry 

pub fn combine_table_entry<'a, F, I>(
    joint_combiner: &F,
    table_id_combiner: &F,
    v: I,
    table_id: &F,
) -> F
where
    F: 'a + Zero + One + Clone,
    I: DoubleEndedIterator<Item = &'a F>,

Let’s say we want to do a lookup in a “vector-valued” table T: Vec<[F; n]> (here I am using [F; n] to model a vector of length n).

For i < n, define T_i := T.map(|t| t[i]).collect(). In other words, the table obtained by taking the ith entry of each element of T.

In the lookup argument, we perform lookups in T by sampling a random challenge joint_combiner, and computing a “combined” lookup table sum_{i < n} joint_combiner^i T_i.

To check a vector’s membership in this lookup table, we combine the values in that vector analogously using joint_combiner.

This function computes that combined value.