Add ad-hoc assumptions about idx in kb, not just idx in e
Add ad-hoc assumptions about idx in kb, not just idx in e

This commit strengths the ad-hoc assumptions added in cca6bc498a27b7f69ce551b7e589bd8f8040e778^..757562a736f7a3fab59c40f1e7ee8a25bb7328e9 to address https://github.com/hakaru-dev/hakaru/issues/59

This commit is motivated by our need to fix https://github.com/hakaru-dev/hakaru/issues/82

zachsully/hakaru,zachsully/hakaru