  A natural p-classes generalization of the eXclusive OR problem, the
subtraction modulo p, where p is prime, is presented and solved using a single
fully connected hidden layer with p-neurons. Although the problem is very
simple, the landscape is intricate and challenging and represents an
interesting benchmark for gradient descent optimization algorithms. Testing 9
optimizers and 9 activation functions up to p = 191, the method converging most
often and the fastest to a perfect classification is the Adam optimizer
combined with the ELU activation function.
