How I work if/else scenarios with probabilities with smart_scripts?

I’m using smart_scripts to fix But at some point in that quest, two things might happen after the Player gossips with an NPC:

  1. A 50% chance that the NPC becomes hostile

  2. if-and-only-if that does not happen, a quest objective must be completed

I looked at the documentation and it says that smart_scripts can handle ‘chances’ but I can’t figure out how to make something else happen if that chance does NOT occur.

Any ideas?