<｜begin▁of▁sentence｜>### Instruction:
Given a graph which you can read from graph_data.gml, and an existing VertexCover object created for that graph, how do you determine the size of the vertex cover?
### Response:
from igraph import Graph, VertexCover

# Load a graph
graph = ig.read('graph_data.gml')

# Assume vc is an existing VertexCover of the graph
vertex_cover = VertexCover(graph, [0, 1, 2])

# Get the size of the vertex cover
size = vertex_cover.size()
print('Size of vertex cover:', size)
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