 
       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()

      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 ['c'], graph.childless()

      test.ok new Set('b', 'a').equals graph.parents('c')
      test.deepEqual ['a'], graph.parents('b')
      test.deepEqual [], graph.parents('a')

      test.ok new Set('b', 'c').equals graph.children('a')
      test.deepEqual ['c'], graph.children('b')
      test.deepEqual [], graph.children('c')

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