Function replacing 

pub fn replacing(limit: WriteLimit, target: &Address) -> GateOutcome

Whether a write that may change a file already there is open under this run’s write limit (crates/kernel/spec/WriteDomain.lean §8-78).

Asked by every write path before it touches a file that exists, or before it starts anything that could: the edit tool’s replacing arm, and a command placed on the host. Whether the target exists is not asked here; a create that finds its name taken is refused by the filesystem at the moment of the write, under either limit.