Return the union of l1 and l2, with a deterministic ordering.
  (Union of python sets does not necessarily have a consisten iteration
  order)
  :param l1: list of items
  :param l2: list of items
  :returns: list containing one copy of each item that is in l1 or in l2

def ordered_union(l1, l2):
  """
  Return the union of l1 and l2, with a deterministic ordering.
  (Union of python sets does not necessarily have a consisten iteration
  order)
  :param l1: list of items
  :param l2: list of items
  :returns: list containing one copy of each item that is in l1 or in l2
  """
  out = []
  for e in l1 + l2:
    if e not in out:
      out.append(e)
  return out