1.6.1 Alternation

The protobuf grammar provides a reserved word, optional, that indicates that the production rule that it refers to may appear once or not at all in a protobuf message. Since Prolog has its own means of alternation, this reserved word is not supported on the Prolog side. It is anticipated that customary Prolog mechanisms for nondeterminism (e.g. backtracking) will be used to generate and test alternatives.