code: def length(self, chain=-1):
          
          if chain is not None:
              if chain < 0:
                  chain = range(self.db.chains)[chain]
              return self._trace[chain].shape[0]
          else:
              return sum([t.shape[0] for t in self._trace.values()])
docs: Return the length of the trace.
  
          :Parameters:
          chain : int or None
            The chain index. If None, returns the combined length of all chains.