From the documentation,

> `VertexContract[g, {v1, v2, ...}]` contracts a collection of vertices `v1`, `v2`, ...  into a single vertex of the graph `g`.

    VertexContract[CycleGraph[6], {3,4}] // EdgeList

    (* {1 <-> 2, 1 <-> 6, 2 <-> 3, 5 <-> 6, 5 <-> 3} *)

Vertices `3` and `4` were contracted into a single vertex now named `3`.  So far so good.

> `VertexContract[g, {{v1, v2, ...}, ...}]` contracts several collections of vertices. 

I am not getting this one.  I expected 

    VertexContract[CycleGraph[6], {{1, 2}, {3, 4}, {5, 6}}]

to contract the pairs `{1,2}`, `{3,4}` and `{5,6}`, each, and get this graph:

    {1 <-> 3, 3 <-> 5, 5 <-> 1}

Instead I get a graph with a single vertex and no edges.  Why?  What does this command actually do?

----

I can do

    Fold[VertexContract[#1, #2] &, g, {{v1, v2, ...}, ...}] 

to achieve the result I expected, but it's pretty slow when there's a lot to contract.