to flatten :l
  if not list? :l [output :l]
  if empty? :l [output []]
  output sentence flatten first :l flatten butfirst :l
end

; using a template iterator (map combining results into a sentence)
to flatten :l
  output map.se [ifelse or not list? ? empty? ? [?] [flatten ?]] :l
end

make "a [[1] 2 [[3 4] 5] [[[]]] [[[6]]] 7 8 []]
show flatten :a