diff --git a/typetapper/hierarchy_graph.py b/typetapper/hierarchy_graph.py
index efcc5d2..9ebe901 100644
--- a/typetapper/hierarchy_graph.py
+++ b/typetapper/hierarchy_graph.py
@@ -101,8 +101,8 @@ class HierarchicalGraph(RelativeAtomGraph):
         # if item is a group, all edges will have two sides
         # if item is an atom, all edges will have one side
         if isinstance(item, RelativeAtom):
-            yield from ((None, (item, succ, key)) for succ in self.__graph.succ[item] for key in succ)
-            yield from (((pred, item, key), None) for pred in self.__graph.pred[item] for key in pred)
+            yield from ((None, (item, succ, key)) for succ, keys in self.__graph.succ[item].items() for key in keys)
+            yield from (((pred, item, key), None) for pred, keys in self.__graph.pred[item].items() for key in keys)
         else:
             yield from ((self.__graph.edges[(item, succ, key)]['prev'], (item, succ, key)) for succ in self.__graph.succ[item] for key in succ)
 
@@ -274,6 +274,14 @@ class HierarchicalGraph(RelativeAtomGraph):
             self.move_node_in(node, moveto)
         self.move_node_in(node, new_parent)
 
+    def create_group(self, nodes: List[RelativeAtomOrGroup], parent: RelativeAtomGroup) -> RelativeAtomGroup:
+        if any(self._parent(node) is not parent for node in nodes):
+            raise ValueError("Not a child of parent")
+        group = self._add_group(parent)
+        for node in nodes:
+            self.move_node_in(node, group)
+        return group
+
     def _parent(self, item: RelativeAtomOrGroup) -> RelativeAtomGroup:
         result = item.parent if isinstance(item, RelativeAtomGroup) else self._atom_parents[item]
         if result is None:
diff --git a/typetapper/relative_graph.py b/typetapper/relative_graph.py
index 0045f55..fcfcc7f 100644
--- a/typetapper/relative_graph.py
+++ b/typetapper/relative_graph.py
@@ -112,9 +112,11 @@ class RelativeAtomGraph:
         relsucc = RelativeAtom(atom=succ, callstack=callstack)
         res = self._add_node(relsucc, path)
         if is_pred:
-            self._add_edge(relsucc, relatom)
+            if not self.__graph.has_edge(relsucc, relatom):
+                self._add_edge(relsucc, relatom)
         else:
-            self._add_edge(relatom, relsucc)
+            if not self.__graph.has_edge(relsucc, relatom):
+                self._add_edge(relsucc, relatom)
         return relsucc if res else None
 
     @staticmethod