There is no direct way to do "else".
The normal way to achieve this in one policy is to have two rules, one which is the "not" of the other.
Or, as the previous poster suggested, you can create an intermediate result and check that in another rule. However, this tends to produce logic which is very hard to read.
By the way, if I understand correctly, not having an "else" clause is quite normal in rules languages.
John D.