ifndef
<br><br>
ifndef is the logical opposite of ifdef. Content between ifndef and endif gets included only if the specified attribute is not set
<div class='example'>
	ifndef::env-github[]
	This content is not shown on GitHub.
	endif::[]
</div>
The syntax of the start directive is ifndef::$attribute[], where $attribute is the name of an attribute.

The ifndef directive supports the same single-line and long-form variants as ifdef.