code: public Set<Plugin> addAdditionalPlugins( Set<Plugin> existing, List<String> additional )
          throws MojoExecutionException
      {
          if ( additional != null )
          {
              for ( String pluginString : additional )
              {
                  Plugin plugin = parsePluginString( pluginString, "AdditionalPlugins" );
  
                  if ( existing == null )
                  {
                      existing = new HashSet<Plugin>();
                      existing.add( plugin );
                  }
                  else if ( !existing.contains( plugin ) )
                  {
                      existing.add( plugin );
                  }
              }
          }
          return existing;
      }
docs: Add the additional plugins if they don't exist yet.
  
  @param existing the existing
  @param additional the additional
  @return the sets the
  @throws MojoExecutionException the mojo execution exception