code: def _MD_hrv_classic_pnn40(X):
      # Proportion of successive differences exceeding 0.04σ (Mietus 2002).
      diffs = np.zeros(len(X) - 1)
      for i in range(len(diffs)):
          diffs[i] = np.abs(X[i + 1] - X[i]) * 1000
  
      nsum = 0
      for diff in diffs:
          if diff > 40:
              nsum += 1
  
      return nsum / len(diffs)
  
imports: import math
  import numpy
  from sktime.utils.numba.njit import njit
label_desc: Use image node_id 28 for calling a global function with example usage: _MD_hrv_classic_pnn40(X) and returns: unknown