The macro tt(DEFINESYMBOL) 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 tt(IFDEF) (see section
ref(ifdef)). 

The corresponding macro tt(UNDEFINESYMBOL) removes the definition of a symbol.

Example:

verb(\
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.))

This produces output similar to:

verb(\
Definining "somesymbol"...
Symbol "somesymbol" is true.
Removing definition of "somesymbol"...
Symbol "somesymbol" is false.)
