fn any_void(&self) -> bool {
            match self {
                Decode::Void => true,
                Decode::Symbol(_,_) => false,
                Decode::Further(map) => map.iter().any(|m| m.any_void()),
            }
        }
Tests to see if the map contains any invalid values.

A correctly initialized map will have no invalid values.
A map with invalid values will be unable to decode some
input byte sequences.
differential-dataflow/src/trace/implementations/huffman_container.rs