I am learning some model based on examples ${((x\_{i1},x\_{i2},....,x\_{ip}),y\_i)}\_{i=1...N}$ using a neural network of Feed Forward Multilayer Perceptron (newff) (using python library neurolab). I expect the output of the NN to be positive for any further simulation of the NN. 
How can I make sure that the results of simulation (testing) of my learned NN are always positive? 
(how I do it in neurolab?)