[Exim] possible conversion issue

In exim 3.36 I had a smartuser driver that had the following condition:

    require_files = \
      ${extract{3}{:}{\
        ${lookup{$domain}dbmnz{/path/to/file.db}{$value}}\
      }}/${substr_0_1:${lc:$local_part}}/${lc:$local_part}/vacation

which worked great. I ran it through the convert4r4 script included with
exim-4.12, and it changed the rule to this:

  require_files = \
      ${extract{3}{:}{\
        ${lookup{$domain}dbmnz{/path/to/file.db}{$value}}\
      }}/${substr_0_1::${lc::$local_part}}/${lc::$local_part}/vacation

Note that it doubled the colons where they occurred in an expansion
operator, but not where it was standing as an extraction delimeter.

Before you tell me to RTFM, I saw the "require_files" warning that was
spit out on STDERR, I just thought maybe the script could be expanded to
look for something like this. =)

--John