def appn(s1, s2):
  """Append two strings, adding a space between them if either is nonempty.

  Args:
      s1 (str): The first string
      s2 (str): The second string

  Returns:
      str: The two strings concatenated and separated by a space.
  """
  if not s1: return s2
  elif not s2: return s1
  else: return s1 + " " + s2