Because properly knowing something is no longer referenced isn't easy. What if you have two objects referencing each other? Extending that line of thinking to resolving any arbitrary data structure, and you'll soon see why the JVM or other garbage collectors are forced to employ far more sophisticated methods of determining what's still needed and what can go.an object (or a value) should be deleted, and leaves that to the implementation (likewise for R5RS).
A language like Java (or its JVM) has many implementations.
Boko Haram Handout/Sahara Reporters via REUTERS Harming the communities Boko Haram began its rebellion in 2009, and has had a very harmful effect on the communities in northeastern Nigeria.
The group has taken thousands of women and young girls, as well as boys and men old enough to fight.
The disappearance has raised fears of another mass kidnapping by Boko Haram.
The terror group took 276 girls from the school in the village of Chibok in April 2014.