theorem Th60:
  for G being _Graph, v being Vertex of G
  holds G.degreeMap().v = G.inDegreeMap().v +` G.outDegreeMap().v
proof
  let G be _Graph, v be Vertex of G;
  thus G.degreeMap().v = v.degree() by Def11
    .= G.inDegreeMap().v +` v.outDegree() by Def12
    .= G.inDegreeMap().v +` G.outDegreeMap().v by Def13;
end;
