foldr (#) v [x0, x1, .. , xn] =
x0 # (x1 # (.. # (xn # v)))
foldl (#) v [x0, x1, .. , xn] =
(((x0 # x1) # ..) # xn) # v 
foldr (#) v [x0, x1, .. , xn] =
x0 # (x1 # (.. # (xn # v)))
foldl (#) v [x0, x1, .. , xn] =
(((x0 # x1) # ..) # xn) # v