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Clearing a symbol known only through a definition

• To: mathgroup at smc.vnet.net
• Subject: [mg66658] Clearing a symbol known only through a definition
• From: Andrew Moylan <andrew.moylan at anu.edu.au>
• Date: Fri, 26 May 2006 04:17:17 -0400 (EDT)
• Sender: owner-wri-mathgroup at wolfram.com

Suppose I have a symbol "a" that is defined to be equal to another 
symbol. (E.g. It might be that a := b.) How can I clear the value of the 
symbol 'pointed to' by "a", without knowing explicity what symbol that is?

Clear[a] won't do, of course.
Clear[Evaluate[a]] won't do, because that will evaluate to 
Clear[Evaluate[b]], which will in turn (in general) evalulate to 
Clear["whatever b evalulates to"].

----
Alternatively, here is the particular problem I want to solve:

For any expression e, either e is a symbol, or Head[e] is, or 
Head[Head[e]] is, etc. Call this the "topmost symbol" in the expression. 
Thus, the topmost symbol in f[x][y][z] is f.

I want to write a function that takes an expression and calls Clear[] on 
its topmost symbol. Can anyone think of a simple way to write such a 
function?

• Follow-Ups:
  • Re: Clearing a symbol known only through a definition
    • From: Andrzej Kozlowski <akoz@mimuw.edu.pl>
  • Re: Clearing a symbol known only through a definition
    • From: "Carl K. Woll" <carlw@wolfram.com>
  • Re: Clearing a symbol known only through a definition
    • From: "Fred Simons" <f.h.simons@tue.nl>
  • Re: Clearing a symbol known only through a definition
    • From: Sseziwa Mukasa <mukasa@jeol.com>