code: 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]
  }
docs: 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