Observe that the type of [fold] is parameterized by _two_ type variables, [X] and [Y], and the parameter [f] is a binary operator that takes an [X] and a [Y] and returns a [Y]. Can you think of a situation where it would be useful for [X] and [Y] to be different

there is no universally accepted definition of art. although commonly used to describe something of beauty, or a skill which produces an aesthetic result, there is no clear line in principle between (say) a unique piece of handmade sculpture, and a mass-produced but visually attractive item. we might say that art requires thought - some kind of creative impulse - but this raises more questions: for example, how much thought is required? if someone flings paint at a canvas, hoping by this action to create a work of art, does the result automatically constitute art?