docs: public long getLastModified( Artifact artifact )
          throws IOException, ArtifactNotFoundException
      {
          Entry entry = backing.get(
              artifact.getGroupId().replace( '.', '/' ) + "/" + artifact.getArtifactId() + "/" + artifact.getVersion()
                  + "/" + artifact.getName() );
          if ( !( entry instanceof FileEntry ) )
          {
              throw new ArtifactNotFoundException( artifact );
          }
          return entry.getLastModified();
      }