join1(delim, vals)
  buf = ''
  for val, i in vals
    if i
      buf += delim + val
    else
      buf += val

join2(delim, vals)
  buf = ''
  for val, i in vals
    buf += i ? delim + val : val

body
  foo join1(' ', 1 2 3)
  foo join1(', ', 1 2 3)
  foo join1(',', 1 2 3)

body
  foo join2(' ', 1 2 3)
  foo join2(', ', 1 2 3)
  foo join2(',', 1 2 3)