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`.
        """
        if isinstance(name, str):
            return self.db.trace(name, chain)
        elif isinstance(name, Variable):
            return self.db.trace(name.__name__, chain)
        else:
            raise ValueError(
                'Name argument must be string or Variable, got %s.' %
                name)