The distinction is similar to persistent data structures and their
builders. Only the MutableFlow can be passed to LogicSystem for
modification; when it's ready, it can be converted into PersistentFlow
and attached to a CFG node.
The result is that the API is cleaner, the implementation is a bit more
neat, and hopefully the use of PersistentHashMap.Builder improves
performance a little. Also the node-to-flow map can now be removed in
favor of just storing PersistentFlow inside a node, seeing as it's
explicitly immutable and all that.
pyos
8d2ac141c3