Riddle 1 Answer

I appear to have left Riddle 1 sitting out there without an official answer for almost seven months now. Sorry about that.

The answer given by Peter Byrne was valid, and essentially the one I was thinking of: while his example dealt with the untyped empty list (), I had the typed empty list `boolean$() in mind.

The insight here is that any and all are forms of min and max; and that min x,y, the min of the concatenation of two lists, is equal to min(min x;min y), the min of their separate mins (and mutatis mutandis for max). For this to work consistently for empty lists, the min of an empty list must be the maximum possible value for that data type (and mutatis mutandis for max).