boolean alias(String src,String target){
  if (src == null)   return false;
  PluginHolder<T> a=registry.get(src);
  if (a == null)   return false;
  PluginHolder<T> b=registry.get(target);
  if (b != null)   return false;
  registry.put(target,a);
  return true;
}

boolean alias ( String src , String target ) { if ( src == null ) return false ; PluginHolder < T > a = registry . get ( src ) ; if ( a == null ) return false ; PluginHolder < T > b = registry . get ( target ) ; if ( b != null ) return false ; registry . put ( target , a ) ; return true ; }
make a plugin available in an alternate name . this is an internal api and not for public use
