Function diff 

pub fn diff(conn: &Connection, t1: &str, t2: &str) -> Result<GraphDiff>

Compare the graph state at two different timestamps.

• added — edges valid at t2 whose (from_id, to_id, relation) triple was not present at t1.
• removed — edges valid at t1 whose triple was not present at t2.
• changed — triples present at both t1 and t2 but with a different id (i.e. the edge was superseded), implying the properties or confidence changed.