DAG bottoms seem to include extra vertices

[aphyr]

I get the impression that DAG bottom sets should be just those vertices with no out edges. However, it looks like they also include nodes in the middle:

(let [g (-> (g/directed-acyclic-graph)
              (g/link :a1 :b)
              (g/link :a2 :b)
              (g/link :b  :c1)
              (g/link :b  :c2)
              (g/link :a3 :c3))]
    (testing "top"
      (is (= #{:a1 :a2 :a3} (datafy (g/top g)))))
    (testing "bottom"
      ; Is the presence of b here a bug, or expected behavior?
      (is (= #{:b :c1 :c2 :c3} (datafy (g/bottom g)))))))

[yuhan0]

Here's an even more minimal repro:

(import '(io.lacuna.bifurcan DirectedAcyclicGraph))

;; Construct the graph  1 → 2 → 3
(-> (new DirectedAcyclicGraph)
    (.link 1 2)
    (.link 2 3)
    (.bottom)
    (.toSet))
;; => #{2 3}

;; Expected?: #{3}

There's an extra factor of ambiguity here with the naming, the root 1 (with no in-edges) could be either said to be the top or bottom node, depending on how one draws the graph.

[yuhan0]

The bug appears to be here

bifurcan/src/io/lacuna/bifurcan/DirectedAcyclicGraph.java

Line 142 in 0e26da7

topPrime = top.add(from);

bifurcan/src/io/lacuna/bifurcan/DirectedAcyclicGraph.java

Line 146 in 0e26da7

bottomPrime = bottom.add(to);

RHS should be operating on topPrime and bottomPrime respectively