overrive bootclasspath using ant

[Dzikus]

Hello,
I need to add some cross-compilation options to my ant buildfiles.
One of them is use another foundation classes.
There is possibility to override (not append) bootclasspath in javac:
-Xbootclasspath/ath

But if I add to my ant script
<bootclasspath><pathelement location="..."/></bootclasspath>
my path is appended to instead of override JDK classes. I cannot find
options that specifies if my path is appended, prepended or overrides.

How build.xml should look like to override bootclasspath?

Thanks in advance
Dominik