How would I do a mixin?

expected-output.txt

version: "3.7"
services:
  cool-container:
    beverages:
    - Mate
    - Coffee
    - Tea
    - Water
    labels:
    - cold
    - ice
    - snow
  hot-container:
    beverages:
    - Coffee
    - Tea
    - Water
    labels:
    - warm
    - lava
    - fire

input.yml

version: '3.7'

---

#@ def beverages( ):
  #@overlay/merge
  beverages:
  - Coffee
  - Tea
  - Water
#@ end

---
#@ load( "@ytt:overlay", "overlay" )
#@overlay/match by=overlay.all
#@overlay/match-child-defaults missing_ok=True
---
#@overlay/assert
version: '3.7'

services:
  
  cool-container:     
    #! How do I overlay the fragment from beverages here? The following don't really work:
    #! #@ overlay.apply( beverages( ) ), this just seems to do nothing
    #! _: #@ template.replace( beverages( ) ), correctly throws an error about a duplicate key
    #!
    #! I've looked in the documentation and the tickets I could find but couldn't find anything
    #! like overlay.insert( ... ), overlay.mixin( ... ) etc.
    beverages:
      - Mate      
    labels:
      - "cold"
      - "ice"
      - "snow"

---
#@ load( "@ytt:overlay", "overlay" )
#@ load( "@ytt:template", "template" )
#@overlay/match by=overlay.all
#@overlay/match-child-defaults missing_ok=True
---
  
#@overlay/assert
version: '3.7'

services:
  hot-container:
    _: #@ template.replace( beverages( ) )
    labels:
      - "warm"
      - "lava"
      - "fire"