 
       test.ok new Set(12, 13, 14, 11, 15).equals graph.childless()
 
+      test.ok new Set(0, 1, 2).equals graph.parents(13)
+      test.ok new Set(2, 0, 10, 7, 9).equals graph.parents(14)
+
+      test.ok new Set(10, 12, 13, 14).equals graph.children(2)
+      test.ok new Set(10, 7, 9, 6).equals graph.children(5)
+
       test.ok new Set().equals graph.whereAllParentsIn []
       test.ok new Set(2).equals graph.whereAllParentsIn [1]
       test.ok new Set(6, 8).equals graph.whereAllParentsIn [5, 3]

      test.deepEqual ['a'], graph.parentless()
      test.deepEqual ['c'], graph.childless()

      test.deepEqual [], graph.whereAllParentsIn []
      test.deepEqual ['b'], graph.whereAllParentsIn ['a']
      test.deepEqual ['c'], graph.whereAllParentsIn ['b']


      test.deepEqual ['a'], graph.parentless()
      test.deepEqual ['b'], graph.childless()

      test.deepEqual ['a'], graph.parents('b')
      test.deepEqual [], graph.parents('a')

      test.deepEqual ['b'], graph.children('a')
      test.deepEqual [], graph.children('b')

      test.deepEqual [], graph.whereAllParentsIn []
      test.deepEqual [], graph.whereAllParentsIn new Set()
