I have a function that returns a xs:boolean. I used xs:value-of to return

	A function that computes atomic values
	should use xsl:sequence rather than
	xsl:value-of

Why?

Why should a function use xsl:sequence?

xsl:value-of is an XSLT 1.0 construct that keeps its semantics in 2.0
and 3.0, it creates a text node with the string value of the selected
expression.

Thus, if you want to return a boolean value computed in the selected
expression, use

as that will return that boolean value and not create and return a text
node with the string representation of the boolean value (that might be
casted back to a boolean if your `xsl:function` also declares
`as="xs:boolean"`).