Appending a yaml string using an overlay

config.yml

#@ load("@ytt:data", "data")
---
#! Input
apiVersion: v1
kind: Secret
metadata:
  name: secret
stringData:
  longYaml: |
    ---
    first: [a, b, c]
    second:
      foo: bar
      ree: baz
    third: true
    

overlay.yml

#@ load("@ytt:overlay", "overlay")
#@ load("@ytt:yaml", "yaml")

#@ def appendingStrings(existing, additional):
#@   return existing+additional
#@ end

#@overlay/match by=overlay.subset({"kind": "Secret", "metadata": {"name": "secret"}})
---
stringData:
  #@overlay/replace via=appendingStrings
  longYaml: |
    fourth: a