How to check if a page is within the rootline of another page in TYPO3 Fluid

typo3-fluid-is-in-rootline.txt

<f:comment>Regular syntax</f:comment>

<v:condition.iterator.contains needle="<the page id>" haystack="{v:page.rootline() -> v:iterator.extract(key: 'uid')}">
    <f:then></f:then>
    <f:else></f:else>
</v:condition.iterator.contains>

<f:comment>Inline syntax</f:comment>

{v:condition.iterator.contains(
    needle: item.uid, 
    haystack: '{v:page.rootline() -> v:iterator.extract(key: \'uid\')}', 
    then: 'foo', 
    else: 'bar' 
)}

Credits to Sven Wappler 
https://www.wapplersystems.de/blog/typo3-pidinrootline-fluid-style/

New URL for credits by Sven Wappler:
https://blog.wappler.systems/typo3-pidinrootline-fluid-style/