... instead of standard lists:• Flatten of nil is X#X (empty difference list).• Flatten of X|Xr where X is a nested list, is Y1#Y4 whereflatten of X is Y1#Y2,flatten of Xr is Y3#Y4 ,and equateY2 and ... removing more and more of theleft-hand part of T2. Eventually, T2’s left subtree is removed completely and theright subtree takes its place. Continuing in this way,T2 shrinks more and more,passing ... LengthLfunction:• Flatten of nil is nil.• Flatten of X|Xr where X is a nested list, is Z whereflatten of X is Y,flatten of Xr is Yr ,and appendY and Yr to get Z.• Flatten of X|Xr where X is not...