*DEFAULT-DATABASE-TYPE* — The default database type to useVariable
The value of this variable is used in calls to
      initialize-database-type and
      connect as the default value of the
      database-type parameter.
If the value of this variable is NIL,
        then all calls to
        initialize-database-type or
        connect will have to specify the
        database-type to use, or a
        general-purpose error will be signalled.