Allow presentation to start with slide number 0

PowerPoint allows, that a presentation starts with number 0. The start value of slide numbering is given by the attribute firstSlideNum of element <p:presentation> in OOXML. Even negative values are allowed in OOXML.

<xsd:attribute name="firstSlideNum" type="xsd:int" use="optional" default="1"/>

The corresponding attribute in ODF is 20.258 style:first-page-number. Here we have the description text

The value of this attribute can be an integer or continue.

But in the schema there is

<attribute name="style:first-page-number">
	<choice>
		<ref name="positiveInteger"/>
		<value>continue</value>
	</choice>
</attribute>

So my suggestion is to allow at least the value 0 too. LibreOffice has not yet implemented the attribute style:first-page-number, but there exists an enhancement request about more flexible page numbering https://bugs.documentfoundation.org/show_bug.cgi?id=115611 . I have no strong opinion, whether negative values should be allowed. For interoperability with pptx format of PowerPoint or SoftMaker, only value 0 is needed.

We have made a similar change in https://issues.oasis-open.org/browse/OFFICE-3923 for the attribute style:page-number, which is used in text documents.