R devel - Jun 2009 - Defining a S4 method for existing function interaction.plot

Dear Members,

i have defined a S4-class and a function for that class wich is to be 
called interaction.plot. Problem: interaction.plot already exists so

setMethod(f = "interaction.plot", signature = "myClass",
definition =
function(x){ return(TRUE) })

   Fehler in conformMethod(signature, mnames, fnames, f, fdef, 
definition) :
   In method for function "interaction.plot": formale Argumente, die
in
der Methodendefinition ausgelassen wurden, k?nnen nicht in der Signatur 
(x.factor = "myClass") sein

won't work because it is already defined.

Is there a solution that keeps the original interaction.plot and lets me 
define a interaction.plot for objects of my S4-Class?

Thank you for your time

Thomas Roth