Strangely enough, IF7 was the first thing to make it "click" for me how declarative logic could be used for general purpose logic programming, which shortly thereafter led me to Definite Clause Grammars, Marcus Triska's work on Definite Clause Grammars[1], and (like this comment) was the gateway drug to (Scryer) Prolog, and Prolog used not just for niche applications but for general purpose computing.

Of course I since realized that was the tip of the iceberg.

[1]: https://youtu.be/CvLsVfq6cks