code: func (p *printer) OneOf(msg *descriptor.DescriptorProto, oneOfIndex int) {
  	of := msg.GetOneofDecl()[oneOfIndex]
  	p.MaybeLeadingComments(of)
  	defer p.open("oneof %s", of.GetName())()
  
  	for i, f := range msg.Field {
  		if f.OneofIndex != nil && int(f.GetOneofIndex()) == oneOfIndex {
  			p.Field(msg.Field[i])
  		}
  	}
  }
docs: // OneOf prints a oneof definition.