<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def eval(self):
 """ Recursively evals the node. Exits with an error if not resolved. """

        Expr.ignore = False
        result = self.try_eval()
        Expr.ignore = True

        return result