mesh generation

I like the new feature to generate complex meshes directly. Here is a small example where I try to generate a stack of layers to be meshed. How do I build the mesh so that it generates the triangles only within the square boundaries?

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from dolfin import *
t1 = 1.0
t2 = 0.5
w = 4

x1 = t1
x2 = x1+t2

vertices = [Point(0.0,0.0), Point(x1,0.0), Point(x1,w/2.0-t2/2.0), Point(x2,w/2.0-t2/2.0),
Point(x2,w/2.0+t2/2.0), Point(x1,w/2.0+t2/2.0), Point(x1,w), Point(0.0,w), Point(0.0,0.0)]
mesh = Mesh()
PolygonalMeshGenerator.generate(mesh, vertices,0.25);
plot(mesh, interactive=True)