anchor.diff:          example = literalExpression ''
             {
               myhost = {
  -              hostNames = [ "myhost" "myhost.mydomain.com" "10.10.1.4" ];
  +              extraHostNames = [ "myhost.mydomain.com" "10.10.1.4" ];
                 publicKeyFile = ./pubkeys/myhost_ssh_host_dsa_key.pub;
               };
  -            myhost2 = {
  -              hostNames = [ "myhost2" ];
  -              publicKeyFile = ./pubkeys/myhost2_ssh_host_dsa_key.pub;
  -            };
  +            "myhost2.net".publicKey = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAILIRuJ8p1Fi+m6WkHV0KWnRfpM1WxoW8XAS+XvsSKsTK";
             }
           '';
         };
  
positive.code:             };
              hostNames = mkOption {
                type = types.listOf types.str;
                default = [];
                description = ''
                  A list of host names and/or IP numbers used for accessing
                  the host's ssh service.
                '';
              };
              publicKey = mkOption {
  
negative.code:               '';
              };
            };
          }));
          description = ''
            The set of system-wide known SSH hosts.
  