> (extend '(3 2 1) g)
((4 3 2 1) (5 3 2 1))
> (extend '(4 3 2 1) g)
()
> (extend '(5 3 2 1) g)
((2 5 3 2 1) (4 5 3 2 1) (6 5 3 2 1))
> (extend-acyclic '(5 3 2 1) g)
((4 5 3 2 1) (6 5 3 2 1))
> (extend-acyclic '(1) g)
((2 1) (3 1))
> (bfs-path 1 1 g)
[image] [image] mcar: contract violation
  expected: mpair?
  given: 1
> (bfs-path 1 1 g)
#t
> (bfs-path 1 1 g)
(1)
> (bfs-path 1 2 g)
[image] [image] ...cheme/graphs.scm:72:2: arity mismatch;
 the expected number of arguments does not match the given number
  expected: 1
  given: 2
> (bfs-path 1 2 g)
[image] [image] mcdr: contract violation
  expected: mpair?
  given: #f
(2 1)
> (bfs-path 1 3 g)
(3 1)
> (bfs-path 1 4 g)
(4 3 1)
> (bfs-path 1 5 g)
(5 3 1)
> (bfs-path 1 6 g)
(6 5 3 1)
> 
> (bfs-path 1 6 g)
((1))
((2 1) (3 1))
((3 2 1) (4 3 1) (5 3 1))
((4 3 2 1) (5 3 2 1) (2 5 3 1) (4 5 3 1) (6 5 3 1))
(6 5 3 1)