Problems with vertices(::Graph) defined in AlgebraicStatistics

[YueRen]

There is a problem with the vertices(::Graph) function in experimental/AlgebraicStatistics/src/GraphicalModels.jl:

function vertices(G::Graph)
  E = [[src(e), dst(e)] for e in edges(G)]
  sort(unique(reduce(vcat, E)))
end

It implicitly assumes that every vertex is connected to an edge, which probably applies to all graphs in algebraic statistics, but need not hold in general.

@bkholler: Would you mind renaming the function with a more appropriate name (e.g., vertices_with_positive_degree)?

Of course, for the sake of consistency, it would be nice to have a general vertices(::Graph) function, but how that function should look like depends on whether vertex sets of polymake graphs need to be numbered 1 to n or whether there be gaps (like {1,2,4}).