Fixing remove id when removing id results in a self-loop

pyzx/basicrules.py

@@ -251,7 +251,11 @@ def remove_id(g: BaseGraph[VT,ET], v: VT) -> bool:
     if not check_remove_id(g, v):
         return False
 
-    v1, v2 = tuple(g.neighbors(v))
+    neighbors = list(g.neighbors(v))
+    if len(neighbors) == 2:
+        v1, v2 = neighbors[0], neighbors[0]
+    else: # self loop
+        v1, v2 = neighbors[0], neighbors[0]
     g.add_edge((v1,v2), edgetype=EdgeType.SIMPLE
             if g.edge_type(g.edge(v,v1)) == g.edge_type(g.edge(v,v2))
             else EdgeType.HADAMARD)

pyzx/graph/multigraph.py

@@ -334,8 +334,12 @@ def connected(self,v1,v2):
     def edge_type(self, e):
         if len(e) == 2:
             edges = list(self.edges(e[0],e[1]))
-            if len(edges) != 1:
-                raise ValueError('Cannot determine edge type')
+            if len(edges) > 1:
+                # if all edges are of the same type, return that type
+                if all(e[2] == edges[0][2] for e in edges):
+                    return edges[0][2]
+                else:
+                    raise ValueError('Cannot determine edge type')
             e = edges[0]
         return e[2]
 

pyzx/rules.py

@@ -791,9 +791,12 @@ def match_ids_parallel(
         v = candidates.pop()
         if phases[v] != 0 or not vertex_is_zx(types[v]) or g.is_ground(v) or g.vertex_degree(v) != 2:
             continue
+        if len(g.incident_edges(v)) != 2: continue
         neigh = g.neighbors(v)
-        if len(neigh) != 2: continue
-        v0, v1 = neigh
+        if len(neigh) == 2:
+            v0, v1 = neigh
+        else:
+            v0, v1 = list(neigh)[0], list(neigh)[0]
         if (g.is_ground(v0) and types[v1] == VertexType.BOUNDARY or
                 g.is_ground(v1) and types[v0] == VertexType.BOUNDARY):
             # Do not put ground spiders on the boundary