you want to check out ssb-sort which has a heads method to see how branch is calculated. Basically looks for the current heads of a causal tree....

the summary of fields I'm using is :

• root : points to the root message in a particular thread
• branch : specifies the heads / most recently seen message(s) in the thread (can be an array if there are parts of the thread which don't know about other parts when they were written
• fork : used to indicate a divergences from the main flow of a thread in 2 ways:
  • "nested reply" : this is for aside comments targeting a specifc message in a thread, so has a root of the thread it's part of and a fork pointing to the "sub-root", the message you're nesting off
  • "hard fork" : this is for signaling that you're creating a different thread but that it came from that original conversation. This is sifnigified by having a fork which points to the prohenitor thread and no root property (because it's a new root for a new fork thread)