modifying thread subject

[Igor Gueths]

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Hi all. This may seem like a stupid question, however I have been wondering for a while as to how the subject line of a thread is changed, yet the way the old subject is tacked onto the new 
one looks exactly the same. For example, subject: abc (was: xyz). Assuming that MUAs are able to insssert things like this automatically, how is Mutt for example able to do this? Perhaps a 
send-hook command? Thanks in advance for any insight into this.
- -- 
Any society that would give up a little liberty to gain a little
   security will deserve neither and lose both.
- -- Benjamin Franklin
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