code: def _is_qrs(self, peak_num, backsearch=False):
          
          i = self.peak_inds_i[peak_num]
          if backsearch:
              qrs_thr = self.qrs_thr / 2
          else:
              qrs_thr = self.qrs_thr
  
          if (i-self.last_qrs_ind > self.ref_period
             and self.sig_i[i] > qrs_thr):
              if i-self.last_qrs_ind < self.t_inspect_period:
                  if self._is_twave(peak_num):
                      return False
              return True
  
          return False
docs: Check whether a peak is a qrs complex. It is classified as qrs
          if it:
          - Comes after the refractory period
          - Passes qrs threshold
          - Is not a t-wave (check it if the peak is close to the previous
            qrs).
  
          Parameters
          ----------
          peak_num : int
              The peak number of the mwi signal to be inspected
          backsearch: bool, optional
              Whether the peak is being inspected during backsearch