cords:
Z    = empty_cord
A    = nonempty_cord(unit_node('a'))
B    = nonempty_cord(list_node('b', []))
AB   = nonempty_cord(branch_node(unit_node('a'), list_node('b', [])))
BA   = nonempty_cord(branch_node(list_node('b', []), unit_node('a')))
ABA1 = nonempty_cord(branch_node(unit_node('a'), branch_node(list_node('b', []), unit_node('a'))))
ABA2 = nonempty_cord(branch_node(branch_node(unit_node('a'), list_node('b', [])), unit_node('a')))
BAB1 = nonempty_cord(branch_node(list_node('b', []), branch_node(unit_node('a'), list_node('b', []))))
BAB2 = nonempty_cord(branch_node(branch_node(list_node('b', []), unit_node('a')), list_node('b', [])))
ABBA = nonempty_cord(branch_node(branch_node(unit_node('a'), list_node('b', [])), branch_node(list_node('b', []), unit_node('a'))))
BAAB = nonempty_cord(branch_node(branch_node(list_node('b', []), unit_node('a')), branch_node(unit_node('a'), list_node('b', []))))

construction: ok

concatenation: ok

equals: ok

identity: ok

length: ok

membership: ok

map: ok

foldl: ok

foldr: ok

condense: ok