SO/a/15327596 builds a degenerate, linear tree

https://stackoverflow.com/a/15327596/849891 builds a degenerate, linear tree.

    mergeAll :: Ord a => [[a]] -> [a]
    mergeAll [] = []
    mergeAll x = head $ mp x

    mp :: Ord a => [[a]] -> [[a]]   -- "mergePairs"
    mp [] = []
    mp [x] = [x]
    mp (x:y:tail) = mp (x+y : mp tail)

    mp [x,y] = mp (x+y : mp []) = mp [x+y] = [x+y]
    mp [z,w,x,y] = mp (z+w : mp [x,y]) = mp [z+w,x+y] = [(z+w)+(x+y)]
    mp [r,s,z,w,x,y] = mp (r+s : [(z+w)+(x+y)]) = [(r+s)+((z+w)+(x+y))]
    mp [t,q,r,s,z,w,x,y] = mp (t+q : [(r+s)+((z+w)+(x+y))]) = [(t+q)+((r+s)+((z+w)+(x+y)))]
    mp [c,d,t,q,r,s,z,w,x,y] = [(c+d)+((t+q)+((r+s)+((z+w)+(x+y))))]
    mp [a,b,c,d,t,q,r,s,z,w,x,y] = [(a+b)+((c+d)+((t+q)+((r+s)+((z+w)+(x+y)))))]

    mergeAll [a,b,c,d,t,q,r,s,z,w,x,y] = (a+b)+((c+d)+((t+q)+((r+s)+((z+w)+(x+y)))))