Reversed name, reversed value

I was able to google how to activate profile when property is NOT PRESENT.
However, I was unable to find any example on how to activate profile when property is NOT EQUAL to some value.
It would be great, if both possibilities were documented here: https://maven.apache.org/pom.html#Activation.

Here's my use case:
<profile>
<id>sign</id>
<activation>
<property>
<name>skipSign</name>
<value>!true</value>
</property>
</activation>
...
</profile>

It seems, It works since maven 3.0 or so (I checked commit history).
Documentation can make that feature to look more "stable".
Thanks!

PS. Here are the sources:
https://git-wip-us.apache.org/repos/asf?p=maven.git;a=blob;f=maven-model-builder/src/main/java/org/apache/maven/model/profile/activation/PropertyProfileActivator.java;hb=HEAD#l64
https://git-wip-us.apache.org/repos/asf?p=maven.git;a=blob;f=maven-model-builder/src/main/java/org/apache/maven/model/profile/activation/PropertyProfileActivator.java;hb=HEAD#l88