How a `K extends keyof T` variable is written in F# (§4.10, the open keyof regime). TypeScript's key variable has no F# counterpart of its own: a bare `'K` would be an unconstrained variable saying nothing about T's keys, and every use of it - including the `T[K]` it selects - would have to widen to obj. The support package carries the idiom instead, so `'K` is not bound at all; its uses are written as one of these.