Error：'function undefined' when calling a new added function in raft.Node interface

[HaoyangShi]

I add a new function imitaing Propose() in raft.Node interface. But there is 'undefined' error when it is called at processInternalRaftRequestOnce() replacing the origin Propose().
I'm quiet confused as the define and use of the new function entirely the same as Propose() and Propose() works well while the new function can not.

I noticed that #5997 may be related but I still don't know how to solve this problem.

// raft/node.go
type Node interface {
        ProposeWithKey(ctx context.Context, data []byte, key []byte) error
        Propose(ctx context.Context, data []byte) error
}

func (n *node) ProposeWithKey(ctx context.Context, data []byte, key []byte) error {
	return n.stepWait(ctx, pb.Message{Type: pb.MsgProp, Entries: []pb.Entry{{Data: data, SfKey: key}}})
}
func (n *node) Propose(ctx context.Context, data []byte) error {
	return n.stepWait(ctx, pb.Message{Type: pb.MsgProp, Entries: []pb.Entry{{Data: data}}})
}

// v3_server.go
func (s *EtcdServer) processInternalRaftRequestOnce(ctx context.Context, r pb.InternalRaftRequest) (*applyResult, error) {
	err = s.r.ProposeWithKey(cctx, data, key) // error: s.r.ProposeWithKey undefined (type raftNode has no field or method sf_ProposeWithKey)
	err = s.r.Propose(cctx, data)
}

[HaoyangShi]

I figure out the problem!
The reason is that the first letter of the function is not capitalized.
The problem is unrelated to etcd. As a beginner in go, I don't know that and find it occasionally.