  We present a space--time model of the collision of two homogeneous, plane
impulsive gravitational waves (each having a delta function profile)
propagating in a vacuum before collision and for which the post collision
space--time has constant curvature. The profiles of the incoming waves are
$k\,\delta(u)$ and $l\,\delta(v)$ where $k, l$ are real constants and $u=0,
v=0$ are intersecting null hypersurfaces. The cosmological constant $\Lambda$
in the post collision region of the space--time is given by $\Lambda=-6\,k\,l$.
