[QUESTION] How to use :filter in `make-process'?

[stardiviner]

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I have an code example try to use :filter handler of make-process.

```
(let ((op-fn (lambda () (message "call the fn"))))
  (make-process
   :name "sleep command test"
   :command (list "sleep" "3")
   :filter `(lambda (proc event)
              (message "event catched!")
              (funcall ,op-fn))
   :buffer "*sleep test*"))
```

I don't know how to use it correctly. Can you give some good examples of using
this `:filter' handler, and explain it a little? 

Thanks in advanced.

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