pub fn write_addr(&mut self, addr: Address<C::F>) -> &mut C::F {
        use vec_map::Entry;
        match self.addr_to_mult.entry(addr.as_usize()) {
            Entry::Vacant(entry) => entry.insert(C::F::zero()),
            Entry::Occupied(entry) => {
                panic!("unexpected entry: addr_to_mult[{:?}] = {:?}", addr.as_usize(), entry.get())
            }
        }
    }
Associate a `mult` of zero with `addr`.

Ensures that `addr` has not already been written to.
crates/recursion/compiler/src/circuit/compiler.rs