templating

What steps did you take:

Input:

#@ def group_alpha():
name: alpha
#@ end
groups: &all_groups
  - #@ group_alpha()
foo:
  groups: *all_groups

Result:

groups:
- null
foo:
  groups:
  - name: alpha

Expected:

groups:
- alpha
foo:
  groups:
  - name: alpha

Expected according to [limitations](https://carvel.dev/ytt/docs/latest/known-limitat

Expected Behaviour

The following should generate a Rancher Rio Service definition:

faas-cli generate --api=services.rio.cattle.io

apiVersion: rio.cat

Cookiecutter knows abbreviations for Github (gh), Bitbucket (bb), and GitLab (gl) projects [...]
https://cookiecutter.readthedocs.io/en/1.7.2/usage.html#works-directly-with-git-and-hg-mercurial-repos-too

$ cookiecutter gh:audreyr/cookiecutter-pypackage