Union of vertex sequences

## Usage

```
# S3 method for igraph.vs
union(...)
```

## Details

They must belong to the same graph. Note that this function has
‘set’ semantics and the multiplicity of vertices is lost in the
result. (This is to match the behavior of the based `unique`

function.)

## See also

Other vertex and edge sequence operations:
`c.igraph.es()`

,
`c.igraph.vs()`

,
`difference.igraph.es()`

,
`difference.igraph.vs()`

,
`igraph-es-indexing`

,
`igraph-es-indexing2`

,
`igraph-vs-indexing`

,
`igraph-vs-indexing2`

,
`intersection.igraph.es()`

,
`intersection.igraph.vs()`

,
`rev.igraph.es()`

,
`rev.igraph.vs()`

,
`union.igraph.es()`

,
`unique.igraph.es()`

,
`unique.igraph.vs()`