# Usage
formula(x, …)
as.formula(object, env = parent.frame())

The generic function formula and its specific methods provide a way of extracting formulae which have been included in other objects.

as.formula is almost identical, additionally preserving attributes when object already inherits from "formula".

object$call[[2]]: object里的call是个列表。call[[2]]：取出列表中的第二项内容. as.formula：把”列表中的第二项内容“用作公式

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