[Exim] Is acl_c9 available in a system filter?

In exim 4.20 there are nine variables that can be set acl_c0 through c9.
My reading lead me to believe that they were available all the way
through routing.

Am I wrong?

If not, why don't I see them in my system filter?

Here is where I set the variable in the acl

  accept  condition = true
          set acl_c9 = "mystring"

This is my system filter:

# Exim filter
testprint "acl_c9=$acl_c9 header_to:=$header_to:"


And this is my debug output (getting pretty good at this debug stuff
:-)

********* from the acl ************
check set = "mystring"
accept: condition test succeeded

********* from the system filter *********
Filter: start of processing
Filter: testprint: acl_c9= header_to:=tom@???

So it looks like the acl set the value, but I don't see it in the system
filter even though I can see header values....