glmmPQL and variance structure

Dear listers,

glmmPQL (package MASS) is given to work by repeated call to lme. In the 
classical outputs glmmPQL  the Variance Structure is given  as " fixed 
weights,  Formula: ~invwt".  The script shows that the function 
varFixed() is used, though the place where 'invwt' is defined remains 
unclear to me.  I wonder if there is an easy way to specify another 
variance structure (eg varPower, etc..), preferably using an lme object 
of the varFunc classes ? Some trials show that the 'weights' argument of 
glmmPQL is just the same as in glm (which is clearly stated in the help) 
and I wonder actually, if not a nonsense, how to pass eg a 'weights' 
arguments as used in lme (eg weights=varPower()) to specify a variance 
function (in the same way as a correlation structure can be passed easy).

Thanks in advance for any hint,

Patrick