code: def number_of(self, axis):
          
          if axis == 'trial':
              return len(self.data)
          else:
              n_trial = self.number_of('trial')
              output = empty(n_trial, dtype='int')
              for i in range(n_trial):
                  output[i] = len(self.axis[axis][i])
  
              return output
docs: Return the number of in one axis, as generally as possible.
  
          Parameters
          ----------
          axis : str
              Name of the axis (such as 'trial', 'time', etc)
  
          Returns
          -------
          int or ndarray (dtype='int')
              number of trial (as int) or number of element in the selected
              axis (if any of the other axiss) as 1d array.
  
          Raises
          ------
          KeyError
              If the requested axis is not in the data.
  
          Notes
          -----
          or is it better to catch the exception?