(patch) add incoming edges

build.sbt

@@ -51,6 +51,7 @@ lazy val testSettings = Seq(
 lazy val `gs-graph` = project
   .in(file("."))
   .aggregate(core, cats, fs2)
+  .settings(noPublishSettings)
   .settings(sharedSettings)
   .settings(testSettings)
   .settings(name := s"${gsProjectName.value}-v${semVerMajor.value}")

modules/core/src/main/scala/gs/graph/v0/Adjacency.scala

@@ -20,6 +20,26 @@ final class Adjacency(val neighbors: Vector[Vector[Vertex]]):
     */
   def at(vertex: Vertex): Vector[Vertex] = neighbors(vertex.ordinal)
 
+  /** Get the vector of _incoming_ [[Vertex]] that point _to_ some input
+    * [[Vertex]].
+    *
+    * @param vertex
+    *   The [[Vertex]] for which to retrieve the incoming vertices.
+    * @return
+    *   The list of [[Vertex]] that have an edge _to_ the input [[Vertex]].
+    */
+  def incoming(vertex: Vertex): Vector[Vertex] =
+    if vertex.ordinal >= neighbors.length then Vector.empty
+    else
+      neighbors.zipWithIndex
+        .filterNot {
+          // ignore the neighbors of the input vertex
+          case (_, index) => index != vertex.ordinal
+        }
+        .filter(_._1.contains(vertex))
+        .map(_._2)
+        .map(Vertex(_))
+
   /** Express this [[Adjacency]] as a vector of [[Edge]].
     *
     * @return