If the technical problem that the skilled person has set himself to solve brings him to the solution step by step, with each individual step being obvious to him in terms of what he has achieved so far and what remains for him to do, the solution is obvious to the skilled person on the basis of the prior art, even if two or more such steps are required, and it does not involve an inventive step (T 623/97, T 911/98, T 558/00, T 1514/05).