Simulates SSI (sequential spatial inhibition) point process.

Syntax
SSI(n, r)

Input
n number of points
r inhibition distance

Output
list of vectors of `x' and `y' coordinates

Details
uses the region set by `ppinit' or `ppregion'.

Side effects
Uses the random number generator.

WARNINGS
will never return if `r' is too large and it cannot place `n' points

See also
`Psim', `Strauss'

Example
towns <- ppinit("towns.dat")
par(pty="s")
plot(Kfn(towns, 10), type="b", xlab="distance", ylab="L(t)")
lines(Kaver(10, 25, SSI(69,1.2)))