def pretty_conjunction(conjunction):
  """Pretty-print a conjunction. Use parentheses as necessary.

  E.g. ["a", "b"] -> "(a & b)"

  Args:
    conjunction: List of strings.
  Returns:
    A pretty-printed string.
  """
  if not conjunction:
    return "true"
  elif len(conjunction) == 1:
    return conjunction[0]
  else:
    return "(" + " & ".join(conjunction) + ")"