a copy of the input FOLD graph, with no array holes.
This method will re-index all component arrays so that there are no arrays with holes (if vertices_ contains vertex 4 but not 3). Arrays with holes are a result of some methods, like subgraph() which is designed so that the user can re-merge the graphs. Alternatively, you can run this method to make a subgraph a valid FOLD object with no holes.
a FOLD object