add a doctest to not blindly trust generic graph

src/sage/graphs/bipartite_graph.py

@@ -574,7 +574,7 @@ def add_vertex(self, name=None, left=False, right=False):
     def add_vertices(self, vertices, left=False, right=False):
         """
         Add vertices to the bipartite graph from an iterable container of
-        vertices. 
+        vertices.
 
         Vertices that already exist in the graph will not be added again.
 
@@ -812,6 +812,10 @@ def add_edge(self, u, v=None, label=None):
             Traceback (most recent call last):
             ...
             RuntimeError: edge vertices must lie in different partitions
+            sage: bg.add_edges([[0, 2]])
+            Traceback (most recent call last):
+            ...
+            RuntimeError: edge vertices must lie in different partitions
             sage: bg.add_edge(0, 3); list(bg.right)
             [1, 3]
             sage: bg.add_edge(5, 6); 5 in bg.left; 6 in bg.right