anchor.diff:  { lib, stdenv, fetchurl, netcat
   
   # Optional packages
  -, systemd ? null, ejabberd ? null, mysql ? null, postgresql ? null, subversion ? null
  +, systemd ? null, ejabberd ? null, mariadb ? null, postgresql ? null, subversion ? null
   , mongodb ? null, mongodb-tools ? null, influxdb ? null, supervisor ? null, docker ? null
   , nginx ? null, s6-rc ? null, xinetd ? null
   
  
positive.code: , getopt
  }:
  
  assert enableMySQLDatabase -> mysql != null;
  assert enablePostgreSQLDatabase -> postgresql != null;
  assert enableSubversionRepository -> subversion != null;
  assert enableEjabberdDump -> ejabberd != null;
  
negative.code:   buildInputs = [ getopt netcat ]
      ++ lib.optional stdenv.isLinux systemd
      ++ lib.optional enableEjabberdDump ejabberd
      ++ lib.optional enableMySQLDatabase mariadb.out
      ++ lib.optional enablePostgreSQLDatabase postgresql
      ++ lib.optional enableSubversionRepository subversion
      ++ lib.optionals enableMongoDatabase [ mongodb mongodb-tools ]
  