You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: op.go
    path of file: ./repos/learning/golang/practice/go-crontab/gopath/src/github.com/coreos/etcd/clientv3
    
    the code of the file until where you have to start completion: // Copyright 2016 The etcd Authors
//
// Licensed under the Apache License, Version 2.0 
    
func opWatch(key string, opts ...OpOption) Op {
	ret := Op{t: tRange, key: []byte(key)}
	ret.applyOpts(opts)
	switch {
	case ret.leaseID != 0:
		panic("unexpected lease in watch")
	case ret.limit != 0:
		panic("unexpected limit in watch")
	case ret.sort != nil:
		panic("unexpected sort in watch")
	case ret.serializable:
		panic("unexpected serializable in watch")
	case ret.countOnly:
		panic("unexpected countOnly in watch")
	case ret.minModRev != 0, ret.maxModRev != 0:
		panic("unexpected mod revision filter in watch")
	case ret.minCreateRev != 0, ret.maxCreateRev != 0:
		panic("unexpected create revision filter in watch")
	}
	return ret
}