def is_connected?(v1, v2)
      raise "not a vertex" unless v1.is_a?(Vertex) && v2.is_a?(Vertex)
      v1.connections.include?(v2)
    end
def is_connected?(v1, v2)
      raise "not a vertex" unless v1.is_a?(Vertex) && v2.is_a?(Vertex)
      v1.connections.include?(v2)
    end
Returns whether two vertices are connected