[Orgmode] Testing presence of a property

[User]

Is there a way to search for headlines which does *not* have set
a particular property? Reading the manual I have the impression
property search has the fixed syntax name=value, so there is no
simple way to search for headers where a property is not yet set
(that is, it has no value).

The only way I found is testing for all possible values, but it's
a bit clumsy (prop<>"value1"+prop<>"value2"+prop<>"value3"...).

Is there a simpler way?