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what they should do, as they always should have, is make the ability to overcome various obstacles an ability inherent to all pokemon who are able to learn the moves. this ability would be shown to the trainer by the gym leader required for the use of the hm normally.
tl;dr beat a gym leader, and all pokemon able to learn the unlocked hm now able to use the overworld ability; surf, fly, etc retconned to tms
