Add code comment for impossible error (#86)

packages/ripple-keypairs/src/secp256k1.ts

@@ -20,6 +20,14 @@ function deriveScalar(bytes, discrim?: number) {
       return key
     }
   }
+  /* This error is practically impossible to reach.
+   * The order of the curve describes the (finite) amount of points on the curve
+   * https://github.com/indutny/elliptic/blob/master/lib/elliptic/curves.js#L182
+   * How often will an (essentially) random number generated by Sha512 be larger than that?
+   * There's 2^32 chances (the for loop) to get a number smaller than the order,
+   * so it's rare that you'll even get past the first loop iteration.
+   * Note that in TypeScript we actually need the throw, otherwise the function signature would be BN | undefined
+   */
   /* istanbul ignore next */
   throw new Error('impossible unicorn ;)')
 }