theorem
  for G being _trivial _Graph, v being Vertex of G holds
    v.inDegree() = G.size() & v.outDegree() = G.size() &
    v.degree() = G.size() +` G.size()
proof
  let G be _trivial _Graph, v be Vertex of G;
  thus v.inDegree() = G.size() & v.outDegree() = G.size() by Th148;
  hence v.degree() = G.size() +` G.size();
end;
