const google.golang.org/protobuf/reflect/protoreflect.Optional

8 uses

	google.golang.org/protobuf/reflect/protoreflect (current package)
		proto.go#L195: 	Optional Cardinality = 1 // appears zero or one times
		proto.go#L203: 	case Optional, Required, Repeated:
		proto.go#L213: 	case Optional:
		proto.go#L227: 	case Optional:

	google.golang.org/protobuf/internal/encoding/tag
		tag.go#L47: 			f.L1.Cardinality = pref.Optional
		tag.go#L163: 	case pref.Optional:

	google.golang.org/protobuf/internal/filedesc
		desc.go#L286: 	return (fd.L0.ParentFile.L1.Syntax == pref.Proto2 && fd.L1.Cardinality == pref.Optional && fd.L1.ContainingOneof == nil) || fd.L1.IsProto3Optional
		desc.go#L398: 	return (xd.L0.ParentFile.L1.Syntax == pref.Proto2 && xd.L1.Cardinality == pref.Optional) || xd.lazyInit().IsProto3Optional

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