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`.

def trace(self, name, chain=-1):
        """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`.
        """
        trace = copy.copy(self._traces[name])
        trace._chain = chain
        return trace