To run caller R:

1. Let A be the following steps:

   1. Fire an event named "a".

2. Queue a microtask, given A.

To run caller S:

1. Let B be the following steps:

   1. Fire an event named "b".

2. Queue a microtask, given B.

To run the same-segment case:

1. Queue a microtask to fire an event named "a".