('for', start, cond, iter, block)

class Node:
    def print(self, indent=0):
        raise NotImplementedError

    def eval(self, env):
        raise NotImplementedError


from collections import namedtuple

@namedtuple
class For(Node):
    start: Node
    cond: Node
    iter: Node
    body: Node

    def print(self, indent=0):
        ...

    def check(self, env):
        pass

    def eval(self, env):
        self.start.eval(env)
        while self.cond.eval(env):
            self.body.eval(env)
            self.iter.eval(env)

n = For(start, cond, iter, block)
n.print()