NixOS environment variables can not be merged?

I tried to install plotinus and got this error:

I don’t know enough of nixos config system, so I have some questions:

  • what’s nixos current behavior for merging config values? I can imagine different merge strategy for different kinds of config key. Please send links to manual’s sections, RFCs or any articles.
  • is there anyway to provide a custom merge function written in nix?