foo <- read.table("squirrel.txt", header = TRUE)
 foo
 bar <- xtabs(count ~ active + passive, data = foo)
 bar

 # Distribution of genital display in a colony of six squirrel monkeys,
 # as reported by Ploog (1967), taken from Fienberg (1980, The Analysis
 # of Cross-Classified Categorical Data, MIT Press), Table 8-2.

 # diagonal elements are "structural zeros" or "fixed zeros"
 # (monkeys do not display to themselves -- the don't have mirrors)
 # xtabs apparently does not have this concept

 # we can fit this with loglin by specifying starting values that have
 # zero on the diagonal
 sss <- (bar + 1)^0
 diag(sss) <- 0
 sss
 lout <- loglin(bar, list(1, 2), start = sss, fit = TRUE)
 lout
 # loglin gets the fit right, I think, but does not count the structural
 # zeros correctly
 # OTOH, glm has no problem (because the structural zeros just are not
 # there in the data to confuse it.
 gout <- glm(count ~ active + passive, family = poisson, data = foo)
 sout <- summary(gout)
 sout
 names(sout)
 sout$deviance
 sout$df.residual
 pchisq(sout$deviance, sout$df.residual, lower.tail = FALSE)

 # This does not get the right degrees of freedom either because of
 # "solutions at infinity".  More on this later.