import sys

class _Exit:
    def __init__(self, code):
        self.code = code

class _Jump:
    def __init__(self, target):
        self.target = target

class _Jmp(_Jump):
    pass

class _Call(_Jump):
    RETURN = object()

class _VM:
    def __init__(self, entry, ctx=None):
        self.current = entry
        self.stack = []
        self.ctx = ctx if ctx is not None else {}

    def start(self):
        while self.current is not None:
            try:
                self.loop()
            except Exception as e:
                print(f"Error: {e}")
                sys.exit(1)
                
    def loop(self):
        action = self.current(self.ctx)

        if isinstance(action, _Jmp):
            self.current = action.target
            return

        if isinstance(action, _Call):
            self.stack.append(self.current)
            self.current = action.target
            return

        if action is _Call.RETURN:
            if not self.stack:
                self.current = None
                return
            self.current = self.stack.pop()
            return

        if isinstance(action, _Exit):
            sys.exit(action.code)

        print(f"Unknown action: {action}")
        sys.exit(1)