== !BeginIf / !EndIf
Encloses directives that will be processed only if a test is true at startup.
This is equivalent to `httpd`'s `IfDefined`

{{{
BeginIf Defined test
    ForceLog "defined"
EndIf
}}}

== !BeginIfNot
Same as above for false values.
This is equivalent to `httpd`'s `IfDefined !parameter-name`

{{{
BeginIfNot Defined test
   ForceLog "not defined"
EndIf
}}}