ant java task bootclasspath prepend

[vnssoftware]

Hello,
what is the corresponding format for:
-Xbootclasspath/path
Specify a colon-separated path of directires, JAR archives, and ZIP
archives to prepend in front of the default bootstrap class path

in the Ant world for java task.
I am only interested in prepend version. Please give an example. I am
not interested in the new bootclasspath altogether.

Thank you