code: def get_value_for_expr(self, expr, target):
          
          if expr in LOGICAL_OPERATORS.values():
              return None
          rvalue = expr['value']
          if rvalue == HISTORICAL:
              history = self.history[target]
              if len(history) < self.history_size:
                  return None
              rvalue = sum(history) / float(len(history))
  
          rvalue = expr['mod'](rvalue)
          return rvalue
docs: I have no idea.