  We consider a porous mediaum flow in which the gas is initially distributed
in the exterior of an empty region (a hole) and study the final stage of the
hole-filling process. Hole-filling is asymptotically described by a
self-similar solution which depends on a constant determined by the initial
configuration. In general, this constant must be found numerically.. Here we
give an example of a one-dimensional symmetric flow where the constant is
obtained explicitly.
