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            <b><a class="bz_bug_link 
          bz_status_UNCONFIRMED "
   title="UNCONFIRMED - FILESAVE Slide numbers are lost when saving as PPTX"
   href="https://bugs.documentfoundation.org/show_bug.cgi?id=117339#c3">Comment # 3</a>
              on <a class="bz_bug_link 
          bz_status_UNCONFIRMED "
   title="UNCONFIRMED - FILESAVE Slide numbers are lost when saving as PPTX"
   href="https://bugs.documentfoundation.org/show_bug.cgi?id=117339">bug 117339</a>
              from <span class="vcard"><a class="email" href="mailto:c3f38p+e9pr063ckikkw@guerrillamailblock.com" title="c3f38p+e9pr063ckikkw@guerrillamailblock.com">c3f38p+e9pr063ckikkw@guerrillamailblock.com</a>
</span></b>
        <pre>It seems noteworthy that there is a field with type "slidenum" in 
ppt/slideMasters/slideMaster1.xml but no (corresponding) entry in
ppt/slide1.xml.

The "solution" in
<a href="https://stackoverflow.com/questions/36190387/how-can-i-add-slide-numbers-to-a-powerpoint-presentation-using-open-xml-net-sdk">https://stackoverflow.com/questions/36190387/how-can-i-add-slide-numbers-to-a-powerpoint-presentation-using-open-xml-net-sdk</a>
indicates there ought to be such a field.

Indeed, adding
<p:sp>
        <p:nvSpPr>
          <p:cNvPr id="127" name="CustomShape 4"/>
          <p:cNvSpPr/>
          <p:nvPr>
            <p:ph type="sldNum"/>
          </p:nvPr>
        </p:nvSpPr>
        <p:spPr>
          <a:xfrm>
            <a:off x="8741520" y="6247440"/>
            <a:ext cx="2840400" cy="472680"/>
          </a:xfrm>
          <a:prstGeom prst="rect">
            <a:avLst/>
          </a:prstGeom>
        </p:spPr>
        <p:txBody>
          <a:bodyPr lIns="0" rIns="0" tIns="0" bIns="0"/>
          <a:p>
            <a:pPr algn="r"/>
            <a:fld id="{BE8C60C3-C7BB-4A63-A38B-623C61B19D0D}" type="slidenum">
              <a:rPr b="0" lang="en-US" sz="1400" spc="-1" strike="noStrike">
                  <a:solidFill>
                  <a:srgbClr val="006699"/>
                  </a:solidFill>
                <a:latin typeface="Arial"/>
                <a:ea typeface="Arial"/>
              </a:rPr>
              <a:t>&lt;number&gt;</a:t>
            </a:fld>
            <a:endParaRPr b="0" lang="en-US" sz="1400" spc="-1"
strike="noStrike">
                  <a:solidFill>
                  <a:srgbClr val="006699"/>
                  </a:solidFill>
              <a:latin typeface="Arial"/>
                <a:ea typeface="Arial"/>
            </a:endParaRPr>
          </a:p>
        </p:txBody>
      </p:sp>

just before </p:spTree> seems to create a slide number which does not vanish on
saving / reopening.</pre>
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