body
  foo join(' ', 1 2 3)
  foo join(', ', 1 2 3)
  foo join(',', 1 2 3)
  foo join(',', 1)
  foo join(',') == null

body
  foo join(', ', 1, 2, 3)
  foo join(', ', one 1, two 2, three 3)

body
  a = 1 2
  b = 3 4
  c = 5 6
  foo: join(', ', 1 2, #fff #000, 5px 6px)
  foo: join(', ', a, b, c)
  foo: join(', ', a b c)
  foo: join(', ', 1, 2, 3, 4, 5, 6)
  foo: join(', ', 1 2 3 4 5 6)