<｜begin▁of▁sentence｜>### Instruction:
Given a graph where some nodes might already be colored, how would you iterate over the remaining nodes in saturation order to extend a graph coloring? As an example, we are using the classic Petersen graph and starting with node 0 colored with color 0.
### Response:
import networkx as nx
from networkx.algorithms.coloring import strategy_saturation_largest_first
# Create a graph
G = nx.generators.classic.petersen_graph()
# Initial coloring (optional)
colors = {0: 0}
# Iterate over nodes in saturation order
for node in strategy_saturation_largest_first(G, colors):
    print('Node:', node, 'Saturation:', len(set(colors.get(neighbor) for neighbor in G.neighbors(node) if neighbor in colors)))
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