code: public function setPsr4($prefix, $paths)
      {
          if (!$prefix) {
              $this->fallbackDirsPsr4 = (array) $paths;
          } else {
              $length = strlen($prefix);
              if ('\\' !== $prefix[$length - 1]) {
                  throw new \InvalidArgumentException("A non-empty PSR-4 prefix must end with a namespace separator.");
              }
              $this->prefixLengthsPsr4[$prefix[0]][$prefix] = $length;
              $this->prefixDirsPsr4[$prefix] = (array) $paths;
          }
      }
docs: Registers a set of PSR-4 directories for a given namespace,
  replacing any others previously set for this namespace.
  
  @param string       $prefix The prefix/namespace, with trailing '\\'
  @param array|string $paths  The PSR-4 base directories
  
  @throws \InvalidArgumentException