def id_index 
      @terms.inject({}) {|sum, t| sum.update(t.id.value => t)}
    end
def id_index 
      @terms.inject({}) {|sum, t| sum.update(t.id.value => t)}
    end
Returns a hash of 'id:012345' => Term

 @return [Hash] a hash of {id => Term} for the file