I'm not saying it's easy, but if you can play what you hum, you don't really have to remember much, depending upon one's repertiore. I mostly play old Country and some Bluegrass. I don't know what the next tune will be, let alone the key. I sorta quickly relearn it, whatever the tune. I listen to/for melodic commonality, rather than differences. This is your own private Magic trick. Do not reveal or discuss this. I found it irritates other players, to reveal certain beloved tunes sound like another. Like you've announced the Emperror is naked. Oh, BTW, to practice this, i used to try to play along with the radio: Find the key, try to play along. Repeat. Repeat. Etc.
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