> I would love to used PBT more, but many tests I write have only one answer per input. Think sum like aggregations.

Not quite sure what you mean by "only one answer per input" (that it's a function, i.e. a 1:1 mapping?), but there are of lots of properties that aggregations might typically need to satisfy, e.g. off the top of my head:

    # Identity element
    forAll(pre, post) {
      assertEqual(
        agg(pre ++ [agg([])] ++ post),
        agg(pre ++ post)
      )
    }

    # Invariant to order
    forAll(elems, seed) {
      assertEqual(
        agg(elems),
        agg(permute(elems, seed))
      )
    }

    # Left-associative
    forAll(xs, ys, zs) {
      assertEqual(
        agg([agg(xs ++ ys)] ++ zs),
        agg(xs ++ ys ++ zs)
      )
    }

    # Right-associative
    forAll(xs, ys, zs) {
      assertEqual(
        agg(xs ++ [agg(yz ++ zs)]),
        agg(xs ++ ys ++ zs)
      )
    }

(FYI these typical properties of a (commutative) monoid, which is an algebraic structure that describes many "aggregation-like" operations)