add.edges adds the specified edges to the graph. The ids of the
vertices are preserved.
add.vertices adds the specified number of isolate vertices to
the graph. The ids of the old vertices are preserved.
delete.edges removes the specified edges from the graph. If a
specified edge is not present, the function gives an error message,
and the original graph remains unchanged.
The ids of the vertices are preserved.
delete.vertices removes the specified vertices from the graph
together with their adjacent edges. The ids of the vertices are
not preserved.