Trying to create new metadata from metadata

[lakonis]

I am trying to create a new key author from existing key authors.

authors:

authors:
  - biography: >-
      my wonderfull biography
      following biography
    forname: Arnaud
    surname: Wormser
  - biography: >-
      my wonderfull biography
      following biography
    forname: Gérard
    surname: Genon

author:

author:
  - biography: >-
      my wonderfull biography
      following biography
    name: Arnaud Genon
  - biography: >-
      my wonderfull biography
      following biography
    name: Gérard Wormser

I understand the get_metadata function, but I don't see how to put new key:value into the metadata block.

I am new to panflute and to pandoc filters in general. I can write Python though, but I'm not familiar with the AST or JSON structure that is manipulated by pandoc filters.