Function may_read 

pub fn may_read(
    reader_building: &Address,
    target: &Address,
    rules: impl FnOnce() -> Result<bool, AxError>,
) -> ReadVerdict

The read bound: whether a resident of reader_building may read target. Its own building is open whole; any other is open unless its rules say it is confidential or cannot be read.

rules answers whether the building holding target is confidential, and is called only when target is outside the reader’s building. It is a closure rather than a value because a read inside one’s own building is the common case and should cost no disk, and because which building holds an address and what its rules say are the city crate’s to answer.