function debuglog (set) {
	if (debugEnv === undefined) {
		debugEnv = process.env.NODE_DEBUG || ''
	}
	const upperSet = set.toUpperCase()
	if (!debugs[upperSet]) {
		if (debugEnv === '*' || new RegExp(`\\b${upperSet}\\b`, 'i').test(debugEnv)) {
			const pid = process.pid
			debugs[upperSet] = (...args) => {
				const msg = util.format(...args)
				console.error('%s %d: %s', upperSet, pid, msg)
			}
		} else {
			debugs[upperSet] = Function.prototype
		}
	}
	return debugs[upperSet]
}
Node's `debuglog` function. Not available on older Node versions, therefore
copied in for convenience:

This is used to create a function which conditionally writes to stderr based
on the existence of a `NODE_DEBUG` environment variable. If the `section`
name appears in that environment variable, then the returned function will
be similar to `console.error()`. If not, then the returned function is a
no-op.

@param  {String} set - the section of the program to be debugged.
@return {Function} - the logging function.
@see https://nodejs.org/api/util.html#util_util_debuglog_section