anchor.diff:      callCabal2nix = name: src: args: self.callCabal2nixWithOptions name src "" args;
   
       # : { root : Path
  +    #   , name : Defaulted String
       #   , source-overrides : Defaulted (Either Path VersionNumber)
       #   , overrides : Defaulted (HaskellPackageOverrideSet)
       #   , modifier : Defaulted
  
positive.code:     # for nix-build or nix-shell to build that package.
      developPackage =
        { root
        , source-overrides ? {}
        , overrides ? self: super: {}
        , modifier ? drv: drv
  
negative.code:            (pkgs.lib.composeExtensions
                (self.packageSourceOverrides source-overrides)
                overrides))
          .callCabal2nix name root {};
        in if returnShellEnv then (modifier drv).env else modifier drv;
  
      ghcWithPackages = selectFrom: withPackages (selectFrom self);
  