still trying to get my head around this and facing obstacles,
most likely by lack of understanding... however

example in

<table>
     <row id="row1" attr1="A" attr2="AA"/>
     <row id="row2"  attr1="C" attr2="CC"/>
     <row id="row3"  attr1="D" attr2="DD"/>
</table>

example out

<document>
     <header>
         <!-- process first row here -->
         <id>row1</id>
         <attr1>A</attr1>
         <attr2>AA</attr2>
     </header>
     <content>
         <!-- process all including first row here -->
         <row id="row1" attr1="A" attr2="AA"/>
         <row id="row2" attr1="C" attr2="CC"/>
         <row id="row3" attr1="D" attr2="DD"/>
     </content>
</document>

Using the accumulator or the xsl:iterate approach allows me to process the header row twice
but I can't seem to figure out to create the header and then continue the iteration in a different construct
it seems I need the header always before the iterator has started

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
	xmlns:xs="http://www.w3.org/2001/XMLSchema"
	xmlns:math="http://www.w3.org/2005/xpath-functions/math"
	exclude-result-prefixes="xs math"
	version="3.0">
	
	<xsl:mode streamable="yes" on-no-match="shallow-copy"/>
	
	<xsl:mode name="header" streamable="yes"/>

	<xsl:output indent="yes"/>
	
	<xsl:template match="table">
		<document>
			<xsl:fork>
				<xsl:sequence>
					<header>
						<xsl:apply-templates select="row[1]/@*" mode="header"/>
					</header>
				</xsl:sequence>
				<xsl:sequence>
					<content>
						<xsl:apply-templates select="row"/>
					</content>
				</xsl:sequence>
			</xsl:fork>
		</document>
	</xsl:template>
	
	<xsl:template match="row/@*" mode="header" expand-text="yes">
		<xsl:element name="{name()}">{.}</xsl:element>
	</xsl:template>
	
</xsl:stylesheet>