/* TASK 3 (7.5pts):

In a similar fashion to map, your next task is to define the filter function

using fold_left.

filter takes in 2 arguments:

1. g - This is a function that decides whether an element in the list is

permitted or not. If the function returns true, then the element is

permitted, otherwise it's not -- it's filtered out.

2. ls - This is the list that passes through filter function g.

Some examples of filter are:

filter(x => x <= 2, List([1,2,3,4])) ------> List([1,2])

filter(x => x % 2 == 0, List([1,2,3,4])) ------> List([2,4])

Again, your constraint is to use fold_left only (no loops!).

*/

const filter = (g, ls) => /** **/ undefined; /** **/