Need help on SiteMinder SAML expression

Hi, 

I have to create send the SAML response with alternate email attribute value, if original mail attribute value is null. 

Ex: If LDAP attribute “mail” value is null, need to send another attribute “altemail” value in response. 

I am new to writing OGNL expressions for federation. Could someone help me on this and suggest to some documents to learn this?

Thanks in advance.