python - In graph-tool, how could I mask a subgraph? -

i create 2 graph via graph-tool library:

g1 = graph(directed=false) g2 = graph(directed=false) g3 = graph(directed=false) 

then:

ug = graph_union(g1, g2) ug = graph_union(ug, g3) 

if want mask g3 , g2 in ug, how do?