import { lensIndex } from "https://deno.land/x/fae@v1.1.1/lensIndex.ts";
Returns a lens whose focus is the specified index.
const headLens = Fae.lensIndex(0)
Fae.view(headLens, ['a', 'b', 'c']) // 'a'
Fae.set(headLens, 'x', ['a', 'b', 'c']) // ['x', 'b', 'c']
Fae.over(headLens, (x: string) => x.toUpperCase(), ['a', 'b', 'c']) // ['A', 'b', 'c']