(flatten '((1 2) (3 4) (5 6)))
result '(1 2 3 4 5 6)
(flatten '((1) ((2 3 (4) (5))) ()))
result '(1 2 3 4 5)
(flatten '((((1))) ((2 3 4)) () () (() ((() (5 ()))))))
result '(1 2 3 4 5)
(foldsum '((1 2) (3 4) (5)))
result 15
(foldsum '((1) ((2 3 (4) (5))) ()))
result 15
(foldsum '((((1))) ((2 3 4)) () () (() ((() (5 ()))))))
result 15