e have created an integer column and arranged for its default
     values to be assigned from a sequence generator.  A <TT
CLASS="LITERAL"
>NOT NULL</TT
>
     constraint is applied to ensure that a null value cannot be
     inserted.  (In most cases you would also want to attach a
     <TT
CLASS="LITERAL"
>UNIQUE</TT
> or <TT
CLASS="LITERAL"
>PRIMARY KEY</TT
> constraint to prevent
     duplicate values from being inserted by accident, but this is
     not automatic.)  Lastly, the sequence is marked as <SP