A vertex sequence is just what the name says it is: a sequence of
vertices. Vertex sequences are usually used as igraph function arguments
that refer to vertices of a graph.

A vertex sequence is tied to the graph it refers to: it really denoted
the specific vertices of that graph, and cannot be used together with
another graph.

At the implementation level, a vertex sequence is simply a vector
containing numeric vertex ids, but it has a special class attribute
which makes it possible to perform graph specific operations on it, like
selecting a subset of the vertices based on graph structure, or vertex
attributes.

A vertex sequence is most often created by the `V()`

function. The
result of this includes all vertices in increasing vertex id order. A
vertex sequence can be indexed by a numeric vector, just like a regular
R vector. See `[.igraph.vs`

and additional links to other
vertex sequence operations below.