Interpreter: Using ~ to refer to before value of a sequence gives error message

[ZoeA]

I have a state variable:

counts : seq of nat 

and I want to refer to the before value of one of the items of the sequence in a post condition. Here's a simple example:

 update:(nat)==>()
 update(n) ==
  (
    counts(n):=counts(n)+1;
        ...
  )
  pre n in set inds counts
  post counts(n) = counts~(n) + 1

This goes through the syntax/type checker fine, but when the operation is called by the interpreter it has an error message:

"Error 4034: Name 'counts~' not in scope in 'H:\Compass\CML\workspace\T243-timing\T243v2.cml' at line 99:19 at in 'H:\Compass\CML\workspace\T243-timing\T243v2.cml' at line 99:19".

I believe this is the correct syntax and there I have no such issue with doing the same thing with a variable of type nat.