think\Loader::addPsr4 PHP Method

    private static function addPsr4($prefix, $paths, $prepend = false)
    {
        if (!$prefix) {
            // Register directories for the root namespace.
            if ($prepend) {
                self::$fallbackDirsPsr4 = array_merge((array) $paths, self::$fallbackDirsPsr4);
            } else {
                self::$fallbackDirsPsr4 = array_merge(self::$fallbackDirsPsr4, (array) $paths);
            }
        } elseif (!isset(self::$prefixDirsPsr4[$prefix])) {
            // Register directories for a new namespace.
            $length = strlen($prefix);
            if ('\\' !== $prefix[$length - 1]) {
                throw new \InvalidArgumentException("A non-empty PSR-4 prefix must end with a namespace separator.");
            }
            self::$prefixLengthsPsr4[$prefix[0]][$prefix] = $length;
            self::$prefixDirsPsr4[$prefix] = (array) $paths;
        } elseif ($prepend) {
            // Prepend directories for an already registered namespace.
            self::$prefixDirsPsr4[$prefix] = array_merge((array) $paths, self::$prefixDirsPsr4[$prefix]);
        } else {
            // Append directories for an already registered namespace.
            self::$prefixDirsPsr4[$prefix] = array_merge(self::$prefixDirsPsr4[$prefix], (array) $paths);
        }
    }