What error should be returned by a krb5_cc implementation which is read-only?

[Jeffrey Altman]

I am finishing my implementation of the krb5_cc implementation for 
accessing the Microsoft Kerberos LSA cache.  In this situation, the 
cache is a read-only device.  Therefore, calls to functions such as:

 krb5_cc_initialize
 krb5_cc_store_cred

should return an error.  Should I create a new error value, 
KRB5_CC_READONLY, or should I
consider this to be an IO error reuse KRB5_CC_IO or KRB5_CC_WRITE?

Thanks.

- Jeff