To be efficient, it is likely that most implementations will rely on internal indices.
But, since this part is likely to be implementation-related rather than enforced by the present specifications, we cannot describe more than some generic methods and a way to provide configuration if needed.
Graph implementation only computes a
structure index which stores neighbors & related edges in a sparse matrix.
Note that, to save some memory sometimes, this index is lazy and will only be computed when necessary (in the case of a multi graph, it might never be computed, for instance).
Forces the computation of the index.
Release the index from memory.