passing an extra argument to an S3 generic

I'm trying to write some functions extending influence measures to
multivariate linear models and also
allow subsets of size m>=1 to be considered for deletion diagnostics. ?I'd
like these to work roughly parallel
to those functions for the univariate lm where only single case deletion
(m=1) diagnostics are considered.

Corresponding to stats::hatvalues.lm, the S3 method for class "lm" objects,

hatvalues <-function (model, ...)

UseMethod("hatvalues")

hatvalues.lm <-

function (model, infl = lm.influence(model, do.coef = FALSE), ? ?...)
{
? ?hat <- infl$hat
? ?names(hat) <- names(infl$wt.res)
? ?hat
}

I have, for class "mlm" objects

hatvalues.mlm <- function(model, m=1, infl=mlm.influence(model, m=m, do.coef
= FALSE), ...)
{
? ?hat <- infl$H
? ?m <- infl$m
? ?names(hat) <- if(m==1) infl$subsets else apply(infl$subsets,1, paste,
collapse=',')
? ?hat
}

where mlm.influence() does the calculations, but also allows the m= argument
to specify subset size.
Yet when I test this I can't seem to pass the m= argument directly, so that
it gets stuffed in to the infl=
call to mlm.influence.

# fit an mlm
library(heplots)
Rohwer2 <- subset(Rohwer, subset=group==2)
rownames(Rohwer2)<- 1:nrow(Rohwer2)
Rohwer.mod <- lm(cbind(SAT, PPVT, Raven) ~ n+s+ns+na+ss, data=Rohwer2)

class(Rohwer.mod)

[1] "mlm" "lm"


## this doesn't work, as I would like it to, calling the hatvalues.mlm
method, but passing m=2:

hatvalues(Rohwer.mod, m=2)

Error in UseMethod("hatvalues") :
?no applicable method for 'hatvalues' applied to an object of class
"c('double', 'numeric')"

I don't understand why this doesn't just call hatvalues.mlm, since
Rohwer.mod is of class "mlm".

# These work -- calling hatvalues.mlm explicitly, or passing the infl=
argument with the call to
# mlm.influence

hatvalues.mlm(Rohwer.mod, m=2)
hatvalues(Rohwer.mod, infl=mlm.influence(Rohwer.mod,m=2))

Can someone help me understand what is wrong and how to make the .mlm method
allow m= to be passed
directly to the infl= computation?

thx,
-Michael

--
Michael Friendly ? ? Email: friendly AT yorku DOT ca
Professor, Psychology Dept.
York University ? ? ?Voice: 416 736-5115 x66249 Fax: 416 736-5814
4700 Keele Street ? ?Web: ? http://www.datavis.ca
Toronto, ONT ?M3J 1P3 CANADA