Brown and Walter provide a wide variety of situations implementing this strategy including a discussion of the development of non-Euclidean geometry. After many years of attempting to prove the parallel postulate as a theorem, mathematicians began to ask "What if it were not the case that through a given external point there was exactly one line parallel to the given line? What if there were two? None? What would that do to the structure of geometry?" (p.47). Although these ideas seem promising, there is little explicit research reported on problem posing.