Adds a vertex to a graph. If called with a vertex, then this vertex is added. If called with a value, then a new vertex is created to hold the value. If-duplicate-do can be one of :ignore, :force, :replace, :replace-value or a function. The default is :ignore.
| add-vertex :before < basic-graph > < basic-vertex > |
| add-vertex < basic-graph > < basic-vertex > |
| add-vertex < basic-graph > < T > |