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The macro <code>DEFINESYMBOL</code> takes one argument, an identifier, which is treated
as a symbol that is set to the logic value `true'. The true-ness or false-ness
of this symbol may subsequently be tested with <code>IFDEF</code> (see section
??). 

The corresponding macro <code>UNDEFINESYMBOL</code> removes the definition of a symbol.

Example:

<pre>
Definining "somesymbol"...
DEFINESYMBOL(somesymbol)
IFDEF(somesymbol)
    (Symbol "somesymbol" is true.)
    (Symbol "somesymbol" is false.)

Removing definition of "somesymbol"...
UNDEFINESYMBOL(somesymbol)
IFDEF(somesymbol)
    (Symbol "somesymbol" is true.)
    (Symbol "somesymbol" is false.)
</pre>

<p>This produces output similar to:
<p><pre>
Definining "somesymbol"...
Symbol "somesymbol" is true.
Removing definition of "somesymbol"...
Symbol "somesymbol" is false.
</pre>

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