ArrayStack is not defined

The error message "ArrayStack is not defined" typically occurs when you are trying to use a variable or object that has not been declared or initialized in your code.

In this case, it seems that you are trying to use a class or object called "ArrayStack" but it has not been defined in your code. You need to make sure that you have imported the necessary modules or defined the class/object before using it in your code.

Here's an example of how to define an ArrayStack class in Python:

class ArrayStack:
    def __init__(self):
        self.items = []

    def push(self, item):
        self.items.append(item)

    def pop(self):
        return self.items.pop()

    def is_empty(self):
        return len(self.items) == 0

    def size(self):
        return len(self.items)

Make sure to import the necessary modules or define the class/object before using it in your code to avoid the "ArrayStack is not defined" error.