code: def result(self, state, row):
          "Place the next queen at the given row."
          col = state.index(None)
          new = state[:]
          new[col] = row
          return nef result(self, state, row):
          "Place the next queen at the given row."
          col = state.index(None)
          new = state[:]
          new[col] = row
          return new
docs: Place the next queen at the given row.