func_code: def union(a, b):
      da = {}
      db = {}
      i = 0
      while i < len(a):
          da[a[i]] = True
          i = i + 1
      i = 0
      while i < len(b):
          db[b[i]] = True
          i = i + 1
      c = []
      for key in da:
          c.append(key)
      for key in db:
          if key not in da:
              c.append(key)
      return c