anchor.diff:  
     config = mkIf cfg.enable {
   
  -    environment.systemPackages = [ pkgs.taskserver ];
  +    environment.systemPackages = [ pkgs.taskserver nixos-taskdctl ];
   
       users.users = optional (cfg.user == "taskd") {
         name = "taskd";
  
positive.code:     ''}
    '';
  
    orgOptions = { name, ... }: {
      options.users = mkOption {
        type = types.uniq (types.listOf types.str);
  
negative.code:       '';
      };
    };
  
    mkShellStr = val: "'${replaceStrings ["'"] ["'\\''"] val}'";
    mkShellName = replaceStrings ["-"] ["_"];
  
    mkSubCommand = name: { args, description, script }: let
      mkArg = pos: arg: "local ${arg}=\"\$${toString pos}\"";
      mkDesc = line: "echo ${mkShellStr "    ${line}"} >&2";
      usagePosArgs = concatMapStringsSep " " (a: "<${a}>") args;
    in ''
      subcmd_${mkShellName name}() {
        ${concatImapStringsSep "\n  " mkArg args}
        ${script}
      }
  
      usage_${mkShellName name}() {
        echo "  nixos-taskdctl ${name} ${usagePosArgs}" >&2
        ${concatMapStringsSep "\n  " mkDesc description}
      }
    '';
  
    mkCStr = val: "\"${escape ["\\" "\""] val}\"";
  
    taskdUser = let
      runUser = pkgs.writeText "runuser.c" ''
        #include <sys/types.h>
        #include <pwd.h>
        #include <grp.h>
        #include <stdio.h>
        #include <stdlib.h>
        #include <errno.h>
        #include <unistd.h>
  
        int main(int argc, char **argv) {
          struct passwd *userinfo;
          struct group *groupinfo;
          errno = 0;
          if ((userinfo = getpwnam(${mkCStr cfg.user})) == NULL) {
            if (errno == 0)
              fputs(${mkCStr "User name `${cfg.user}' not found."}, stderr);
            else
              perror("getpwnam");
            return EXIT_FAILURE;
          }
          errno = 0;
          if ((groupinfo = getgrnam(${mkCStr cfg.group})) == NULL) {
            if (errno == 0)
              fputs(${mkCStr "Group name `${cfg.group}' not found."}, stderr);
            else
              perror("getgrnam");
            return EXIT_FAILURE;
          }
          if (setgid(groupinfo->gr_gid) == -1) {
            perror("setgid");
            return EXIT_FAILURE;
          }
          if (setuid(userinfo->pw_uid) == -1) {
            perror("setgid");
            return EXIT_FAILURE;
          }
          argv[0] = "taskd";
          if (execv(${mkCStr taskd}, argv) == -1) {
            perror("execv");
            return EXIT_FAILURE;
          }
          /* never reached */
          return EXIT_SUCCESS;
        }
      '';
    in pkgs.runCommand "taskd-user" {} ''
      cc -Wall -std=c11 "${runUser}" -o "$out"
    '';
  
    subcommands = {
      list-users = {
        args = [ "organisation" ];
  
        description = [
          "List all users belonging to the specified organisation."
        ];
  
        script = ''
          legend "The following users exist for $organisation:"
          ${pkgs.findutils}/bin/find \
            "${cfg.dataDir}/orgs/$organisation/users" \
            -mindepth 2 -maxdepth 2 -name config \
            -exec ${pkgs.gnused}/bin/sed -ne 's/^user *= *//p' {} +
        '';
      };
  
      list-orgs = {
        args = [];
  
        description = [
          "List available organisations"
        ];
  
        script = ''
          legend "The following organisations exist:"
          ${pkgs.findutils}/bin/find \
            "${cfg.dataDir}/orgs" -mindepth 1 -maxdepth 1 \
            -type d
        '';
      };
  
      get-uuid = {
        args = [ "organisation" "user" ];
  
        description = [
          "Get the UUID of the specified user belonging to the specified"
          "organisation."
        ];
  
        script = ''
          for uuid in "${cfg.dataDir}/orgs/$organisation/users"/*; do
            usr="$(${pkgs.gnused}/bin/sed -ne 's/^user *= *//p' "$uuid/config")"
            if [ "$usr" = "$user" ]; then
              legend "User $user has the following UUID:"
              echo "$(${pkgs.coreutils}/bin/basename "$uuid")"
              exit 0
            fi
          done
          echo "No UUID found for user $user." >&2
          exit 1
        '';
      };
  
      export-user = {
        args = [ "organisation" "user" ];
  
        description = [
          "Export user of the specified organisation as a series of shell"
          "commands that can be used on the client side to easily import"
          "the certificates."
          ""
          "Note that the private key will be exported as well, so use this"
          "with care!"
        ];
  
        script = ''
          if ! subcmd_quiet list-users "$organisation" | grep -qxF "$user"; then
            exists "User $user doesn't exist in organisation $organisation."
          fi
  
          uuid="$(subcmd_quiet get-uuid "$organisation" "$user")" || exit 1
  
          cat <<COMMANDS
          taskdatadir="\''${TASKDATA:-\$HOME/.task}"
          umask 0077
          mkdir -p "\$taskdatadir/keys"
          cat > "\$taskdatadir/keys/public.cert" <<EOF
          $(cat "${cfg.dataDir}/keys/user/$organisation/$user/public.cert")
          EOF
          cat > "\$taskdatadir/keys/private.key" <<EOF
          $(${pkgs.gnused}/bin/sed -ne '/^---* *BEGIN /,/^---* *END /p' \
            "${cfg.dataDir}/keys/user/$organisation/$user/private.key")
          EOF
          cat > "\$taskdatadir/keys/ca.cert" <<EOF
          $(cat "${cfg.dataDir}/keys/ca.cert")
          EOF
          task config taskd.certificate -- "\$taskdatadir/keys/public.cert"
          task config taskd.key         -- "\$taskdatadir/keys/private.key"
          task config taskd.ca          -- "\$taskdatadir/keys/ca.cert"
          task config taskd.credentials -- "$organisation/$user/$uuid"
          COMMANDS
        '';
      };
  
      add-org = {
        args = [ "name" ];
  
        description = [
          "Create an organisation with the specified name."
        ];
  
        script = ''
          if [ -e "orgs/$name" ]; then
            exists "Organisation with name $name already exists."
          fi
          ${taskdUser} add org "$name"
        '';
      };
  
      add-user = {
        args = [ "organisation" "user" ];
  
        description = [
          "Create a user for the given organisation and print the UUID along"
          "with the client certificate and key."
        ];
  
        script = ''
          if subcmd list-users "$organisation" | grep -qxF "$user"; then
            exists "User $user already exists in organisation $organisation."
          fi
          ${taskdUser} add user "$organisation" "$user"
          ${genClientKey}
        '';
      };
  
      add-group = {
        args = [ "organisation" "group" ];
  
        description = [
          "Create a group for the given organisation."
        ];
  
        script = ''
          if [ -e "orgs/$organisation/groups/$group" ]; then
            exists "Group $group already exists in organisation $organisation."
          fi
          ${taskdUser} add group "$organisation" "$group"
        '';
      };
    };
  
    mkCase = name: { args, ... }: let
      mkPosArg = pos: const "\"\$${toString (pos + 1)}\"";
      cmdArgs = concatImapStringsSep " " mkPosArg args;
    in ''
      ${name})
        if [ $# -ne ${toString ((length args) + 1)} ]; then
          echo "Wrong number of arguments to ${name}." >&2
          echo >&2
          usage_${mkShellName name}
          exit 1
        fi
        subcmd "${name}" ${cmdArgs};;
    '';
  
    nixos-taskdctl = pkgs.writeScriptBin "nixos-taskdctl" ''
      #!${pkgs.stdenv.shell}
      export TASKDDATA=${mkShellStr cfg.dataDir}
  
      quiet=0
      # Deliberately undocumented, because we don't want people to use this as
      # it's only used in and specific to the preStart script of the Taskserver
      # service.
      if [ "$1" = "--service-helper" ]; then
        quiet=1
        exists() {
          exit 0
        }
        shift
      else
        exists() {
          echo "$@" >&2
          exit 1
        }
      fi
  
      legend() {
        if [ $quiet -eq 0 ]; then
          echo "$@" >&2
        fi
      }
  
      subcmd() {
        local cmdname="''${1//-/_}"
        shift
        "subcmd_$cmdname" "$@"
      }
  
      subcmd_quiet() {
        local prev_quiet=$quiet
        quiet=1
        subcmd "$@"
        local ret=$?
        quiet=$prev_quiet
        return $ret
      }
  
      ${concatStrings (mapAttrsToList mkSubCommand subcommands)}
  
      case "$1" in
        ${concatStrings (mapAttrsToList mkCase subcommands)}
        *) echo "Usage: nixos-taskdctl <subcommand> [<args>]" >&2
           echo >&2
           echo "A tool to manage taskserver users on NixOS" >&2
           echo >&2
           echo "The following subcommands are available:" >&2
           ${concatMapStringsSep "\n     " (c: "usage_${mkShellName c}")
                                           (attrNames subcommands)}
           exit 1
      esac
    '';
  
    ctlcmd = "${nixos-taskdctl}/bin/nixos-taskdctl --service-helper";
  
  in {
  
  