docs: def singletrial(num_trials, skipstep=1):
      """ Single-trial cross-validation schema
  
      Use one trial for training, all others for testing.
  
      Parameters
      ----------
      num_trials : int
          Total number of trials
      skipstep : int
          only use every `skipstep` trial for training
  
      Returns
      -------
      gen : generator object
          the generator returns tuples (trainset, testset)
      """
      for t in range(0, num_trials, skipstep):
          trainset = [t]
          testset = [i for i in range(trainset[0])] + \
                    [i for i in range(trainset[-1] + 1, num_trials)]
          testset = sort([t % num_trials for t in testset])
          yield trainset, testset