Disable validate on previous state

Question

How to disable validate when go to previous state?

<view-state id="step2State" view="q_step2" model="questionModel">
<binder>
    <binding property="abcDate" required="true" />
    <binding property="defDate" required="true" />
</binder>
<transition on="prev" to="step1State" />
<transition on="next" to="step3Action" />
</view-state>

and when user to go prev transition - WebFlow require to fill abcDate and defDate.