commit the current statements from add()

def execute(self, **kwargs):
    """ commit the current statements from add()
    """
    assert self.resp is None, "Transaction already committed"
    try:
      self.resp = self.db.tx(list(self.edn_iter), **kwargs)
    except Exception:
      self.resp = False
      raise
    else:
      self.resolve()
      self.adds = None
      self.tmpents = None
    return self.resp