code: function( term ) {
      
      var document_count
      var appearances = 1
      
      for( key in this.documents ) {
        this.documents[ key ][ term ]
          ? appearances++ : 0
      }
      
      document_count = Object.keys( this.documents )
      document_count = document_count.length + 1
      
      return Math.log( document_count / appearances )
      
    }
docs: [idf description]
  @param  {String} term
  @return {Number}