def make_connection(v1, v2)
      raise "not a vertex" unless v1.is_a?(Vertex) && v2.is_a?(Vertex)
      raise "already connected" if is_connected?(v1, v2)
      # Make new edge
      edge = Edge.new(v1, v2)
      # Connect the two using the vertex method "connect"
      v1.connect(v2, edge)
      v2.connect(v1, edge)

      # Add to edge catalog
      @edges << edge
      edge
    end
def make_connection(v1, v2)
      raise "not a vertex" unless v1.is_a?(Vertex) && v2.is_a?(Vertex)
      raise "already connected" if is_connected?(v1, v2)
      # Make new edge
      edge = Edge.new(v1, v2)
      # Connect the two using the vertex method "connect"
      v1.connect(v2, edge)
      v2.connect(v1, edge)

      # Add to edge catalog
      @edges << edge
      edge
    end
Make an edge between two vertices