Hledger.Data.Balancing.
Both Hledger.Data.Transaction and Hledger.Data.Journal are massive
module with many things in them. Placing the balancing functions, which
are conceptually related, into a separate module helps keep things more
modular.
It also reduces the risk of import cycles, as right now balancing
functions cannot depend on any functions defined outside of
Hledger.Data.Transaction or Hledger.Data.Journal, respectively, if those
modules require basic transaction or journal functions.
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