There shouldn't be two encodings for unset. Many places have to know whether a type is a container to append the right unset value even though it had to know nothing else about the type.

The modified tag encoding could be

0 = nil
1 = zero-length primitive (e.g., "" or int 0)
N/2 even = primitive of length (N/4)
N/2 odd = container of length (N/4)-1

This will change the zng format and should be part of zng-beta.