[R] constraining covariance structure for random effects in lme

[Nisha C. Gottfredson]

I would like to impose a unique structure on the random effects 
covariance matrix in a multilevel model (i.e., a structure that is not 
included in the standard pdMat classes listed in Table 4.3 of Pinheiro 
and Bates' lme book).  Is there any way to manually impose equality 
constraints on the elements of the random effects covariance matrix?

Thank you.