toggling a property to control target execution

[Lou Colon]

Is there a way to unset a property?
I am trying to use the conditional target execution based on a property.

 The problem is that once the property has been set (to any value) I have
not been able to return to the previously set state. For example,

<ant target="doSomething">
        <!- does nothing since prop is not set ->
</ant>
<ant target="doSomething">
        <property name="prop" value="anything"/>
        <!- executes doSomething since prop is set ->
</ant>
<ant target="doSomething">
        <!- executes doSomething since prop was already set
             but I want to turn it off ->
</ant>

Thanks,
Lou