code: public function set($prefix, $paths)
      {
          if (!$prefix) {
              $this->fallbackDirsPsr0 = (array) $paths;
          } else {
              $this->prefixesPsr0[$prefix[0]][$prefix] = (array) $paths;
          }
      }
docs: Registers a set of PSR-0 directories for a given prefix,
  replacing any others previously set for this prefix.
  
  @param string       $prefix The prefix
  @param array|string $paths  The PSR-0 base directories