Letters Are Local Names
This video presents the same text shown beside it, spoken and on screen. It adds nothing the text does not say.
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A letter in a formula is a name for something, hired by a nearby sentence and holding its meaning only within the discussion that introduced it.
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One unit's b names a branching factor; another discussion's b may name something else entirely — no connection implied.
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A formula with an unhired letter is not deep; it is incomplete — hunt backward for the hiring sentence.