S4: inheritance of validity methods?

Dear Developers,

In current implementation of validity method, objects are first
coerced to  superclass (slots are striped). Thus, it is not possible
to write validity  method which would perform some checks on children
slots.

Say, I want to check if number of slots in a class is equal to "n":

setClass("A", representation(a="numeric", n="integer"),
          prototype=list(a=12, n=1L),
          validity=function(object){
              if(length(slotNames(object))!=object at n+1) paste("Number
              of  slots must be ", object at n)
              else TRUE
          })


setClass("B", representation(b="numeric"), contains="A",
          prototype=list(a=12, b=14, n=2L))


new("B", a=11, b=33)
Error in validObject(.Object) :
   invalid class "B" object: Number of slots must be  2

Error, because an object of class "A" is passed to validObject with
one  slot "b" removed and n=2.

Is were a work around for this, or I am just doomed to write the same
validity method for each children class?

Many thanks,
Vitalie.