Redundant double negation

not(not(x)) negates x and then negates the result — a round trip that just coerces x to a boolean. It is exactly boolean(x), and wherever nothing but a truth is taken it is exactly x. Written as a double negation it reads as a puzzle: the reader has to cancel the two nots in their head to see what it means, and then wonder whether the author meant something subtler than what the cancellation leaves.

Those places are the @test of an xsl:if or an xsl:when, the use-when that decides whether an element is compiled at all, an operand of and or or, the argument of another not(), the condition of an if expression and the body of a satisfies: each takes the effective boolean value of what stands there, so the two calls compute what the place computes next anyway. Everywhere else the boolean value is the value of the expression, and boolean(x) says it once instead of twice. An argument that binds loosely needs brackets when it stands as an operand, since a or b written where not(not(a or b)) and @c stood would be read as a or (b and @c).

Incorrect:

<xsl:if test="not(not(@enabled))">
<xsl:value-of select="@a and not(not(@enabled))"/>
<xsl:value-of select="not(not(@enabled))"/>

Correct:

<xsl:if test="@enabled">
<xsl:value-of select="@a and @enabled"/>
<xsl:value-of select="boolean(@enabled)"/>