Re: Explanation Request for the Function set-transient-map in subr.el

[xenodasein]

Thanks for your clarification on isearch issue.

>> ((eq t keep-pred)
>>   (let ((mc (lookup-key map (this-command-keys-vector))))
>>   ;; If the key is unbound `this-command` is
>>   ;; nil and so is `mc`.
>>     (and mc (eq this-command mc))))
>>

I'm still not clear why exactly this expression is used though: (eq 
this-command mc).
What are the possible values of mc here on the last line?