How to combine/merge multiple graphs using LEMON (or BGL)

Question

I am looking for a way to merge multiple Graphs into a single Graph using LEMON. If it can not be done, I'd also be interested in solutions using Boost Graph Library.

The background is, that I want to define smaller structural units as graphs, annotate them with properties and add multiple copies of them together to create more complex structures.

In the example below, I have 3 graphs g1,g2. and g3 consisting of a single vertex. I want to combine them into a single graph and be able to add edges to connect the vertices inside the resulting graph. However, I don't know how to specify the copyGraph function correctly so that all vertices of g1, g2, and g3 end up in the same graph at the end. Also, if g1 etc. contained multiple connected vertices, these edges should remain intact.

The example also contains a fourth graph gTest which looks like the result I want to achieve. However, gTest here is created from scratch and not by combining the previously exisiting graphs. This is for demonstration/clarification and produces the following graphic:

To compile the code using g++, use

g++ -lemon main.cpp

#include <lemon/list_graph.h>
#include <lemon/graph_to_eps.h>

int main()
{
    using namespace lemon;
    typedef dim2::Point<int> Point;

    ListGraph g1; 
    ListGraph::NodeMap<Point> coords1(g1);
    ListGraph::Node n1 = g1.addNode();
    coords1[n1] = Point(10,20);

    ListGraph g2; 
    ListGraph::NodeMap<Point> coords2(g2);
    ListGraph::Node n2 = g2.addNode();
    coords2[n2] = Point(20,20);

    ListGraph g3; 
    ListGraph::NodeMap<Point> coords3(g3);
    ListGraph::Node n3 = g3.addNode();
    coords3[n3] = Point(15,25);


    // pseudo-code of the copy-operation:
    // copyGraph(g1, g3);
    // copyGraph(g2, g3);
    //  
    // g3.addEdge(n1,n3)
    // g3.addEdge(n2,n3)



    // Result should look similar to what the following produces
    ListGraph gTest;
    ListGraph::NodeMap<Point> coordsTest(gTest);
    ListGraph::Node n1Test = gTest.addNode();
    coordsTest[n1Test] = Point(10,20);
    ListGraph::Node n2Test = gTest.addNode();
    coordsTest[n2Test] = Point(20,20);
    ListGraph::Node n3Test = gTest.addNode();
    coordsTest[n3Test] = Point(15,25);

    ListGraph::Edge e1Test = gTest.addEdge(n3Test, n1Test);
    ListGraph::Edge e2Test = gTest.addEdge(n3Test, n2Test);

    graphToEps(gTest, "gTest.eps")
        .coords(coordsTest)
        .nodeTexts(coordsTest)
        .run();

    return 0;
}