import { prefix } from "https://deno.land/x/pazza@v0.3.0/mod.ts";
Execute the "prefix" parse first. If it succeeds, discard its result and execute the following normal parser.
If any of both parsers fails, the whole parser will fail.
const parser = prefix(char("<"), digit());
parser("<5").output === "5";
parser("[5").ok === false;
parser("<a").ok === false;