LET’S TRY ONE
The rule says: If a badge has a blue triangle, it opens this gate. No rule about other badges is supplied.
What does the blue triangle establish under this rule?
It is sufficient for opening; the text does not establish that it is necessary.
The rule guarantees opening for a blue-triangle badge but does not exclude other opening badges.
Learning steps & teaching tips
Use the exact direction of a stated rule to distinguish a guarantee from a requirement, without reversing an implication.
- Identify the condition and result in the exact wording.
- Test whether the condition guarantees the result, is required for it, or both.
- Check a proposed inference against the rule instead of assuming its converse.
Try it together
Write three invented access rules using if, only if and if and only if. Make a case that satisfies each rule and one inference the wording does not justify.
If A then B guarantees B when A holds; B only if A makes A required. Do not silently turn either rule into an equivalence.