The <idno> content model is currently text only. This is plainly ridiculous because an identification number can contain all sorts of characters. Especially because the locations <idno> is available has been decided to be increased recently. This might include characters that are not yet in Unicode, especially if <idno> is being used to record complete fantastical or mythological identification numbers. If changing this content model to include model.gLike it would also potentially make sense to add model.glossLike in order to give <idno> a <desc> child to document the nature of this <idno>.