One summary per group

A non-aggregate head argument splits the rows into groups: one summary per distinct value.

For example, we can count how many literals are negated in each clause C in our CNF example:

count_neg(C, count(*)) :- literal(C, _, 0).

This works as follows:

1. Evaluate the body of the predicate; the result is a set of rows, one per negated literal.
2. For each C, add a row (C, n) to count_neg, where n is the number of those rows with that C.