def LexBFS(G):
"""Find lexicographic breadth-first-search traversal order of a graph.
G should be represented in such a way that "for v in G" loops through
the vertices, and "G[v]" produces a sequence of the neighbors of v; for
instance, G may be a dictionary mapping each vertex to its neighbor set.
Running time is O(n+m) and additional space usage over G is O(n).
"""
P = PartitionRefinement(G)
S = Sequence(P, key=id)
while S:
set = S[0]
v = arbitrary_item(set)
yield v
P.remove(v)
if not set:
S.remove(set)
for new,old in P.refine(G[v]):
S.insertBefore(old,new)
