I would rather say that everything is a snapshot - presumably I am working at a particular timestamp level - i.e. I won't get some facts that were valid at Time

I would rather say that everything is a snapshot - presumably I am working at a particular timestamp level - i.e. I won't get some facts that were valid at Timestamp A and some that were valid at Timestamp B. My question is if/how you do conditional updates. Say I'm storing some particular concept C as a collection of facts, and I want to update fact C-1 to '15' if fact C-2 is '0'. In an RDBMS I might select for update fact C-2 to make sure it didn't concurrently change underneath me while I'm making the change - is there an equivalent in Datomic? I understand that under normal circumstances reads are completely decoupled from writes, but what if I want to make a write if and only if a certain state holds?