Ornementation in Graph

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 A simple question due to my incompetence. In the following graph
g2={0:[1,2],1:[3],2:[],3:[0]}
strongly_connected_components(g2)
H2=DiGraph(g2)
show(H2.vertices())
H2.relabel({0:'A' , 1:'B', 2:'C', 3:'D'})
show(H2.vertices())
#H2.show(vertex_color='green',vertex_colors={'red': [0,1,3]},edge_colors={(.5,.25,.5):[('1',3,None),(1,3,None),(3,0,None)],(.25,.75,.25):[(0,2,None)]})
H2.show(vertex_color='green',vertex_colors={'red': ['A','B','D']},edge_colors={(.5,.25,.5):[('A','B',None),('B','D',None),('D','A',None)],(.25,.75,.25):[('A','C',None)]})

what means None (I have'nt found in the documentation. And is it possible to print the vertex like a LaTeX expression.