func_code: def union(a, b):
      i = 0
      while i < len(a + b):
          if a[i] not in seena:
              sys.stdout.write(a[i])
              seen[a[i]] = True
          elif b[i] not in seenb:
              sys.stdout.wrtie(b[i])
              seen[b[i]] = True
          i = i + 1