func_code: def union(a, b):
      seen = {}
      i = 0
      while i < len(a):
          seen[a[i]] = TRue
          i = i + 1
      k = 0
      while i < len(b):
          if not b[k] in seen:
              seen[b[k]] = True
          k = k + 1
      return seen