Repeating units with repetition ranges - Frequency variation
A sequence of ranges to specify the repetition can also be used in a special group called repeating unit with repetition ranges.
For example:
     
        
Here the repetition range is "3,5-7". The repetition count for the included structure can be: 3,5,6 or 7.
Syntax of the repetition ranges
The repetition ranges consist of ranges and/or numbers separated by commas. E.g 3, 5-7
To draw a repeating unit with repetition ranges
-     Draw the structure. 
-     Click the Group tool on the toolbar, and select the structure to be included in the range. 
-     In the "Create Group" dialog window choose the type "Repeating unit with repetition ranges" from the "Type" list. 
-     Set the repetition ranges. 
-     Click OK.