def add_vertex(vertex, name: nil)
      name ||= callback(vertex, :name)
      insert_vertex(vertex, name)
      define_currified_methods(vertex, :vertex) if @currify
      callback(vertex, :added_to_graph, self)
      self
    end
def add_vertex(vertex, name: nil)
      name ||= callback(vertex, :name)
      insert_vertex(vertex, name)
      define_currified_methods(vertex, :vertex) if @currify
      callback(vertex, :added_to_graph, self)
      self
    end
Add a vertex into the graph

 If a name: is given, or the vertex responds to :name,
 it will be registered by name in the graph