[R] "locked environment"

[Heywood, Giles]

I have a class defined as an extension to matrix:
fooCLASS <- setClass("foo",representation("matrix"))

and define a method for a generic function (no problem):
setMethod("diff",signature(x="foo"),diff.foo <- function(x){x})

but I get a different behaviour for another generic function:
setMethod("lag",signature(x="foo"),lag.foo <- function(x){x})

Error in assign(".packageName", pkg, env) : 
can't add bindings to a locked environment

I did not get this behaviour under 1.5.1, it ran fine - any pointers?

platform i386-pc-mingw32
arch     i386           
os       mingw32        
system   i386, mingw32  
status                  
major    1              
minor    7.0            
year     2003           
month    04             
day      16             
language R              


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