code: def trace(self, name, chain=-1):
          
          trace = copy.copy(self._traces[name])
          trace._chain = chain
          return trace
docs: Return the trace of a tallyable object stored in the database.
  
          :Parameters:
          name : string
            The name of the tallyable object.
          chain : int
            The trace index. Setting `chain=i` will return the trace created by
            the ith call to `sample`.