In a phonological rule formula, you can right-click a natural class and select occurrence values, such as Occurs exactly once. You can alternatively use the Set Occurrence dialog box to choose specific maximum and minimum values. The maximum value is <infinite>. The minimum value is 0 (zero).
In the Phonological Rules pane, click the desired phonological rule.
In the Phonological Rule pane Rule Formula field, right-click a natural class in an environment, and then click Set occurrence (min. and max.).
The Set Occurrence dialog box appears.
Select a different Maximum value and Minimum value, as needed.
Click OK.
The values set appear as superscript and subscript number or symbols on the right side of the environment.
For example, means this environment can occur zero or more (infinite) times.
Parsing words (Hermit Crab parser)