How to choose options from categorical slots in stories?

I have the slot defined in the domain file as:

    type: rasa.shared.core.slots.CategoricalSlot
    initial_value: null
    auto_fill: true
    influence_conversation: true
    - mild
    - moderate
    - severe

I want my story to be defined as one line of action for both mild and moderate categories in severity slot and a different story line if I have ‘severe’ as the slot input.

Below is the pseudocode of the expected outcome:

 if severity=='Mild' or 'Moderate:
        story 1:
else if severity=='Severe'
        story 2:

The current story has the following code:

  - intent: severity
  - slot_was_set:
    - severity: Severe
  - action: utter_emergency

Looking forward to some guidance.

Take a look at the or story example in the docs here