import { flatten } from "https://deno.land/x/30_seconds_of_typescript@v1.0.1/util.ts";
Flattens an array up to the specified depth.
Use recursion, decrementing depth by 1 for each level of depth.
Use Array.prototype.reduce() and Array.prototype.concat() to merge elements or arrays.
Base case, for depth equal to 1 stops recursion.
Omit the second argument, depth to flatten only to a depth of 1 (single flatten).