[Zope] Test of existance of attributes

[Sin Hang Kin]

The code below do not works as I through: when the attribute expiredate is
not defined, it raise an error instead of display the sequence-item.  How
can I detect the existence of the attribute before comparision?

Rgs,

Kent Sin

PS the code :

<dtml-in "objectValues(['DTML Document'])" sort="id" skip_unauthorized>

<dtml-if "expiredate and (expiredate - ZopeTime() ) >= 0.0 and not deleted">

<h2><dtml-var title_or_id></h2>
<dtml-if "AUTHENTICATED_USER.has_permission( 'Change DTML Documents',
_['sequence-item'])">
<a href="manage_edit">Edit</a>
</dtml-if>
<dtml-if "(ZopeTime() - _['bobobase_modification_time']) > Newdate">
<em>New</em>
</dtml-if>
<dtml-var sequence-item fmt="structured-text">
</dtml-if>
</dtml-in>