code: def keys(self):
          " Returns the keys of all the elements."
          if self.ndims == 1:
              return [k[0] for k in self.data.keys()]
          else:
              return list(self.data.keys()f keys(self):
          " Returns the keys of all the elements."
          if self.ndims == 1:
              return [k[0] for k in self.data.keys()]
          else:
              return list(self.data.keys())
docs: Returns the keys of all the elements.