[xsl] How to test whether boolean parameter has been set?

Subject: [xsl] How to test whether boolean parameter has been set?
From: "Jorge . chocolate.camera@xxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>
Date: Mon, 29 Oct 2018 09:07:59 -0000
In XSLT2, how would you test whether a boolean global parameter optionally
empty

    <xsl:param name="THING" as="xs:boolean?"/>

has been given a value at runtime?

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