def add(fact_name, fixtures)
      @index_data[fact_name] ||= {}
      fixtures.each do |fixture|
        fact_value = get_fact(fixture, fact_name)
        next if fact_value.nil?
        @index_data[fact_name][fact_value] ||= []
        @index_data[fact_name][fact_value] << fixture.hostname
      end
    end
def add(fact_name, fixtures)
      @index_data[fact_name] ||= {}
      fixtures.each do |fixture|
        fact_value = get_fact(fixture, fact_name)
        next if fact_value.nil?
        @index_data[fact_name][fact_value] ||= []
        @index_data[fact_name][fact_value] << fixture.hostname
      end
    end
Constructor.

 data     - A Hash of existing index data.
 filename - Optionally, a String with a file name to write the index to
 Add a fact to the index. If the fact already exists in the index, this will overwrite it.

 fact_name - A String with the name of the fact
 fixtures  - An Array with fact fixtures (must respond to .facts and .hostname)