FOM: Refutation Theorem Proving and Natural Deduction?

[Raatikainen Panu A K]

Steve Stevenson wondered whether tree or tableau method 
necessarily assumes classical logic. 

Now there is an intuitionistic version of the methdod, although it is 
admittedly somewhat less elegant. 
See J.L. Bell and m. Machover: A Course in Mathematical Logic, 
chapter 9.

-Panu Raatikainen