Truncate the trace array to some index.

        :Parameters:
        index : int
          The index within the chain after which all values will be removed.
        chain : int
          The chain index (>=0).

def truncate(self, index, chain):
        """
        Truncate the trace array to some index.

        :Parameters:
        index : int
          The index within the chain after which all values will be removed.
        chain : int
          The chain index (>=0).
        """
        self._trace[chain] = self._trace[chain][:index]