source.br

lazy leaf:
  my _; _
  yields :Leaf
  yields :Nil; :Nil

lazy node:
  my a; _
  yields :Node
  yields (bintree a); (bintree a)

defn bintree:
  my n
  if n == 1:
    return (leaf ())
  or:
    return (node n-1)

defn print_bintree:
  my tree
  t, l, r = tree! ()
  if t == :Leaf:
    print "Leaf"
  or:
    print "Node("
    print_bintree l
    print ","
    print_bintree r
    print ")"

print_bintree (bintree 4)