code: public function add($prefix, $paths, $prepend = false)
      {
          if (!$prefix) {
              if ($prepend) {
                  $this->fallbackDirsPsr0 = array_merge(
                      (array) $paths,
                      $this->fallbackDirsPsr0
                  );
              } else {
                  $this->fallbackDirsPsr0 = array_merge(
                      $this->fallbackDirsPsr0,
                      (array) $paths
                  );
              }
  
              return;
          }
  
          $first = $prefix[0];
          if (!isset($this->prefixesPsr0[$first][$prefix])) {
              $this->prefixesPsr0[$first][$prefix] = (array) $paths;
  
              return;
          }
          if ($prepend) {
              $this->prefixesPsr0[$first][$prefix] = array_merge(
                  (array) $paths,
                  $this->prefixesPsr0[$first][$prefix]
              );
          } else {
              $this->prefixesPsr0[$first][$prefix] = array_merge(
                  $this->prefixesPsr0[$first][$prefix],
                  (array) $paths
              );
          }
      }
docs: Registers a set of PSR-0 directories for a given prefix, either
  appending or prepending to the ones previously set for this prefix.
  
  @param string       $prefix  The prefix
  @param array|string $paths   The PSR-0 root directories
  @param bool         $prepend Whether to prepend the directories