How to perform a DISTINCT in Pig Latin on a subset of columns?

Question

I would like to perform a DISTINCT operation on a subset of the columns. The documentation says this is possible with a nested foreach:

You cannot us

Answer 1

unique_A = FOREACH (GROUP A BY (a1, a2, a3)) {
    limit_a = LIMIT A 1;
    GENERATE FLATTEN(limit_a) AS (a1,a2,a3,a4);
};