[math-fun] Tiling the Plane with 5-, 6- and 7-gons only

A colleague wants a computer to define a 2D-graph, a tiling of the plane if you will, which only has pentagons, hexagons and heptagons. He's a chemist, which may give someone a clue what this is about, but that someone is not me. I don't suppose the n-gons can all be convex (but don't have a proof of that) but concave n-gons are not necessarily an issue. Anyone got an algorithm or even better, a program. My colleague did have the suggestion of just tiling the plane with triangles and then knocking sides out, but the problem then becomes one of inspecting what is left to see that it meets the requirements. Thanks - Guy