interface KindFn
implements Kind
import { type KindFn } from "https://deno.land/x/fun@v2.0.0/fn.ts";
Specifies Fn as a Higher Kinded Type, with covariant parameter A corresponding to the 0th index of any Substitutions and a contravariant parameter D corresponding to the 0th index of any Substititions. The Fn KindFn is unique in that it constrains the Fn type to taking a single argument for the purposes of type substitution while the implementations of Fn combinators such as map, flatmap, etc are mostly variadic (multiple arguments).