How to use formulas for multivariate clustered data?

Dear R experts,

I wrote already a couple of weeks ago but did not get any reply. Therefore
I'll rephrase my question and hope some of you can give me some answers or
some advice where better to look for an answer.

In general I'm looking for information on how to program functions using
formulas in order to write a nonparametric multivariate regression function
with possible clustering of the data.

I would assume that the formulas for my function would have the form
y1 + y2 + y3 ~ x | z
or
Y ~ x | z,

where y1, y2 and y3 are continuous response variables, or Y the matrix of
responses and x an explaining variable and z a factor indicating the cluster
membership.

I'm looking for general information about writing functions using formulas.
But more concrete I would need to know what would be the best way to make
sure that the response should be multivariate and numeric? What is the best
way to define the hierarchical structure, is it the "|" or is there another,
better way? How can I restrict the number of hierarchical levels?
And how to use in this context, the data, subset, weights and na.action
arguments?

Thanks in advance,

Klaus

--
Klaus Nordhausen
Researcher
Tampere School of Public Health
FIN-33014 University of Tampere

phone: ?+358 3 3551 4153
fax: ? ?+358 3 3551 4150
e-mail: Klaus.Nordhausen at uta.fi