@CanIgnoreReturnValue
    public Builder<E, O> add(E protoEnum, O objectEnum) {
      fromProtoEnumMap.put(protoEnum, objectEnum);
      toProtoEnumMap.put(objectEnum, protoEnum);
      return this;
    }
Adds bidirectional conversion mapping between {@code protoEnum} and {@code objectEnum}.
java_src/src/main/java/com/google/crypto/tink/internal/EnumTypeProtoConverter.java