Subant: Calls a given target in another build file

There is a very useful ant task, the subant task. It allows to call a target in an external build file.
It requires this syntax:

<subant buildfile="../subproject/build.xml"
  target="mytarget" 
  buildpath="../subproject" />

The best way to use this task, is to invoke it in another target in the current build file.