(<literal>&lt;</literal>,
   <literal>&gt;</literal> or <literal>~</literal>) can be stored as well.
   (Certainty indicators are ignored by all the built-in operators, however.)
   <xref linkend="seg-repr-table"/> gives an overview of allowed
   representations; <xref linkend="seg-input-examples"/> shows some
   examples.
  </para>

  <para>
   In <xref linkend="seg-repr-table"/>, <replaceable>x</replaceable>, <replaceable>y</replaceable>, and
   <replaceable>delta</replaceable> denote
   floating-point numbers.  <replaceable>x</replaceable> and <replaceable>y</replaceable>, but
   not <replaceable>delta</replaceable>, can be preceded by a certainty indicator.
  </para>

  <table id="seg-repr-table">
   <title><type>seg</type> External Representations</title>
   <tgroup cols="2">
    <tbody>
     <row>
      <entry><literal><replaceable>x</replaceable></literal></entry>
      <entry>Single value (zero-length interval)
      </entry>
     </row>
     <row>
      <entry><literal><replaceable>x</replaceable> .. <replaceable>y</replaceable></literal></entry>
      <entry>Interval from <replaceable>x</replaceable> to <replaceable>y</replaceable>
      </entry>
     </row>
     <row>
      <entry><literal><replaceable>x</replaceable> (+-) <replaceable>delta</replaceable></literal></entry>
      <entry>Interval from <replaceable>x</replaceable> - <replaceable>delta</replaceable> to
      <replaceable>x</replaceable> + <replaceable>delta</replaceable>
      </entry>
     </row>
     <row>
      <entry><literal><replaceable>x</replaceable> ..</literal></entry>
      <entry>Open interval with lower bound <replaceable>x</replaceable>
      </entry>
     </row>
     <row>
      <entry><literal>.. <replaceable>x</replaceable></literal></entry>
      <entry>Open interval with upper bound <replaceable>x</replaceable>
      </entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <table id="seg-input-examples">
   <title>Examples of Valid <type>seg</type> Input</title>
   <tgroup cols="2">
    <colspec colname="col1" colwidth="1*"/>
    <colspec colname="col2" colwidth="2*"/>
    <tbody>
     <row>
      <entry><literal>5.0</literal></entry>
      <entry>
       Creates a zero-length segment (a point, if you will)
      </entry>
     </row>
     <row>
      <entry><literal>~5.0</literal></entry>
      <entry>
       Creates a zero-length segment and records
       <literal>~</literal> in the data.  <literal>~</literal> is ignored
       by <type>seg</type> operations, but
       is preserved as a comment.
      </entry>
     </row>
     <row>
      <entry><literal>&lt;5.0</literal></entry>
      <entry>
       Creates a point at 5.0.  <literal>&lt;</literal> is ignored but
       is preserved as a comment.
      </entry>
     </row>
     <row>
      <entry><literal>&gt;5.0</literal></entry>
      <entry>
       Creates a point at 5.0.  <literal>&gt;</literal> is ignored but
       is preserved as a comment.
      </entry>
     </row>
     <row>
      <entry><literal>5(+-)0.3</literal></entry>
      <entry>
        Creates an interval <literal>4.7 .. 5.3</literal>.
        Note that the <literal>(+-)</literal> notation isn't preserved.
      </entry>
     </row>
     <row>
      <entry><literal>50 .. </literal></entry>
      <entry>Everything that is greater than or equal to 50</entry>
     </row>
     <row>
      <entry><literal>.. 0</literal></entry>
      <entry>Everything that is less than or equal to 0</entry>
     </row>
     <row>
      <entry><literal>1.5e-2 .. 2E-2 </literal></entry>
      <entry>Creates an interval <literal>0.015 .. 0.02</literal></entry>
     </row>
     <row>
      <entry><literal>1 ... 2</literal></entry>
      <entry>
       The same as <literal>1...2</literal>, or <literal>1 .. 2</literal>,
       or <literal>1..2</literal>
       (spaces around the range operator are ignored)
      </entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   Because the <literal>...</literal> operator is widely used