logcontract

I have a problem manipulating a certain term. This problem can be 
reduced to the following basic problem:

assume(a<1);
log(a-1)-log(-1);
logcontract(%);

I would expect this to give the desired result log(1-a), but it doesn't. 
It just doesn't do anything. Any ideas or explanations?

Greetings from M?nster, Germany
Wolfgang Hugemann