cross-nested random structure in glmmPQL

Dear members,
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I'm trying to fit a GLMM using glmmPQL with a cross-nested random structure. Basically, I have perfectly crossed observations of whether there is a relation between two people (sender and receiver of relation) and I'd like to do a random structure where the relations to an from the same actor are related.
If A and B are the random effects for specific sender and receiver the covariance matrix should look like this:
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sigma_A^2*I????? sigma_AB*I
sigma_AB*I?????? sigma_B^2*I
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where I is an n x n identity matrix.
The code I have so far has the random structure of:
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const <-? factor(rep(1, length(data$y)))
mod <- glmmPQL(y ~ as.factor(variableS) + as.factor(variableR), 
               random = list(const = pdBlocked(list(pdIdent(~1),pdIdent(~sender-1),pdIdent(~receiver-1)))),
               data=data, family=binomial)

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I think this code only leads to a diagonal covariance matrix and doesn't consider the covariance sigma_AB I'd like to model. Do you have any suggestions how I have to change the random part of my formula in order to get the right covariance matrix?
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Any help is highly appreciated!
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Thank you,
Carrie