NuSMV执行flatten_hierarchy报错：‘x-1’未定义问题咨询

Let's break down why you're seeing this error and how to fix it. The "undefined" error for x-1 typically stems from NuSMV's static type checking or expression resolution rules, even though your logic seems correct at first glance. Here are the most likely causes and solutions:

1. Bounded Integer Type Compatibility Issue #

Your variable x is declared as 0..3, a bounded integer type. While x-1 stays within 0..3 when x>0, some older NuSMV versions have strict static checking that may not recognize the conditional guard (status = no_empty) & (x>0) as sufficient to guarantee the result is within bounds.

Fix:
Change x to an unbounded integer type and add an invariant to enforce its original range:

MODULE main
VAR
    status:{empty, no_empty};
    x : integer;  // Switch to integer type
ASSIGN
    init(status):= empty;
    init(x):=0;
    next(status):= case
        (status = empty): no_empty;
        (status = no_empty) & (x=0): empty;
        TRUE: status;
    esac;
    next(x):= case
        (status = empty): x+3;
        (status = no_empty) & (x>0): x-1;
        TRUE: x;
    esac;
INVAR x >= 0 & x <= 3;  // Enforce original bounds with an invariant

2. Operator Parsing Ambiguity #

Even with parentheses, NuSMV's parser might misinterpret the conditional expression in edge cases. Using the short-circuit logical AND (&&) instead of the standard & can help the parser correctly prioritize your conditions.

Fix:
Modify the next(x) case guard to use short-circuit AND:

next(x):= case
    (status = empty): x+3;
    (status = no_empty) && (x>0): x-1;  // Use short-circuit AND
    TRUE: x;
esac;

3. Outdated NuSMV Version #

Older NuSMV releases (pre-2.6) have known quirks with arithmetic operations on bounded integers. If you're using an outdated version, upgrading to the latest stable build will likely resolve the issue.

4. Explicit Expression Formatting #

Sometimes, adding explicit parentheses around arithmetic expressions can help the parser resolve them correctly, especially if there's a hidden parsing quirk.

Fix:
Rewrite x-1 with clear spacing and parentheses:

(status = no_empty) & (x>0): (x - 1);

After trying any of these fixes, re-run flatten_hierarchy—the error should no longer occur.

内容的提问来源于stack exchange，提问作者Gold