How to preserve private lines of code

Hi, I have a private and a public repository that share part of the codebase. The workflow is one-way only: it syncs from the public to the private repository.

```
core.workflow(
name = "pull",
origin = git.github_origin(
url = sourceUrl,
ref = "main",
),
destination = git.github_pr_destination(
url = destUrl,
destination_ref = "main",
pr_branch = "copybara_from_public_repo",
title = "Copybara - code sync from public repo",
body = "This PR is auto-generated by Copybara.",
integrates = [],
),

origin_files = glob(
include = filesToSync,
exclude = excludedFiles
),

destination_files = glob(
include = filesToSync,
exclude = excludedFiles
),

authoring = authoring.pass_thru("author "),
transformations = [
metadata.restore_author("ORIGINAL_AUTHOR", search_all_changes = True),
metadata.use_last_change(author=True, message=True, default_message=None),
],
mode = ITERATIVE
)
```

The issue arises when a private function is added to the private codebase. Copybara tries to delete it because it is not present in the public codebase, and the file is included in the sync.

How can I preserve the private codebase while keeping the file in sync?

Thanks!