When assembling multiple parts of a configuration (like a part for the whole company, then a part for each application), we sometimes need to define a macro "locally" (for an application) until it's defined "globally" (at the company level). Then, when defined globally (and deployed), the local version shouldn't be used anymore.

It would be very handy to have a directive allowing to check if a macro is already defined, like IfDirective. Examples of use:

<IfMacro Macro1>
 Use Macro1 ...
</IfMacro>

<IfMacro !Macro2>
 <Macro Macro2 ...>
  ...
 </Macro>
</IfMacro>
Use Macro2 ...