Optional parameter "filter" needs clarification.

3.1.2.1 Properties

Says: "the properties listed MUST be returned". What happens if a property is listed but the object-type of some of the results do not define such a property?
o For example, I call getChildren with filter=cmis:path" and the folder contains both cmis:document and cmis:folder base types? Those objects which are not cmis:folder do not have a cmis:path property, so is this a failure (throw exception) as I can not satisfy the MUST called out in 3.1.2.1 (that is, I MUST return cmis:path for the cmis:document objects, but this is not possible - so throw an exception? - seems harsh).

How does filter work?
o Should it return only those properties which are listed, when the object actually has such a property (and thus just quietly ignore filter properties that don't apply)? Note that under some circumstances (e.g. in the above cmis:path example), this implies that some objects may be returned without any properties at all.
o Should it allow only properties which are in the intersection of all possible (or actual) object-types being returned?