[R] constructing grouped data object when grouping is nested within a fixed factor

[E. Corda]

       [this was erronously sent to "owner-R-help" {which is the
        list maintainer, a person -- manually re-posted, MM.]
Dear all,
Does anyone know how to construct a grouped data object where the grouping
variable is nested within a fixed factor? 
Indeed I have three zones which I consider as the levels of a fixed factor,
and in each zone I have 5 circuits on which several observations were made.
My grouping variable is the circuit nested within zone. So is it possible
to specify this although the zone is considered as fixed? After that I
would like to specify a mixed-effects model using nlme library.
Another question linked to the first one: in such a case, is it equivalent
to create a new CIRCUIT variable numbered from 1 to 15, and to specify it
as the grouping variable?
Thanks in advance for any help.
Eve CORDA 
Office national de la chasse et de la faune sauvage
5, rue de Saint Thibault
SAINT-BENOIST
78610 AUFFARGIS
BP 20 - 78612 LE PERRAY EN YVELINES Cedex
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Tel : +33 (0)1.30.46.60.64
Fax : +33 (0)1.30.46.60.99
Email : e.corda at oncfs.gouv.fr
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