例えばmergeをする関数など

for inはfor-inのループじゃない。(for ofがjsのfor-in)

merge = (xs,ys) ->
  n = xs.length

  if typeof ys.length is "number"
    xs[n++] = e for e in ys
  else
    xs[n++] = ys[j++] while ys[j] isnt undefined

  xs.length = n
  xs

####
a = merge [1,2,3], [10,20,30]
console.dir a