Nested is nested type.

Intentionaly complicated message type to cover many features of Protobuf.

NumericEnum is one or zero.

DeepEnum is one or zero.

paths: \"f.a\" paths: \"f.b.d\" Here `f` represents a field in some root message, `a` and `b` fields in the message found in `f`, and `d` a field found in the message in `f.b`.