 Solution

Use the same step from part (a):The right fold obeys groupFold [] = [] and groupFold (x:xs) = prependRun x (groupFold xs), exactly the recursive equations already proved. Both implementations give the bottom value for an entirely undefined input spine. Their equal behaviour on finite partial lists, together with definable continuity, establishes equality on infinite lazy lists as well. The helper's irrefutable binding is what lets the first run be produced without demanding that all subsequent runs already exist.