The pipeline
<p:declare-step xmlns:p="http://www.w3.org/ns/xproc" version="3.0">
<p:output port="result">
<p:text-tail count="1" name="head">
<p:with-input>
<p:inline content-type="text/plain">This
is
just
some
arbitrary
text
with
nine
lines.</p:inline>
</p:with-input>
</p:text-tail>
<p:text-count name="counter">
<p:wrap-sequence wrapper="text">
<p:with-input pipe="@head">
</p:wrap-sequence>
<p:add-attribute attribute-name="counter">
<p:with-option name="attribute-value" select="/*:result/text()" pipe="@counter">
</p:add-attribute>
</p:declare-step>
Schematron validation
<s:schema xmlns="http://www.w3.org/1999/xhtml" xmlns:s="http://purl.oclc.org/dsdl/schematron" queryBinding="xslt2">
<s:pattern>
<s:rule context="/">
<s:assert test="text">Root element is not 'text'.</s:assert>
<s:assert test="text/@counter='1'">Attribute 'counter' is not '1'.</s:assert>
<s:assert test="text/text()='lines.
'">Text content of 'text' is wrong.</s:assert>
</s:rule>
</s:pattern>
</s:schema>
