Function diff 

pub fn diff(
    t: &Transition,
    config_diff: Option<&ConfigDiff>,
    evaluated: bool,
    removed: &[(UnitId, Sensitivities)],
    each_known: bool,
    tables: &Tables,
) -> ApprovalDiff

The diff a generation transition would apply; empty for any other. config_diff and removed (the units it would remove or stop, from removed) come from the derived facts.