function
boot.janet on line 1632, column 1
(flatten-into into xs)
Takes a nested array (tree) `xs` and appends the depth first
traversal of `xs` to array `into`. Returns `into`.
(flatten-into @[1 2 3] @[[4 5 6] @[7 8 9] @[10 11 12]]) # => @[1 2 3 4 5 6 7 8 9 10 11 12]