tt(DEFINESYMBOL) expects two arguments. An identifier, which is the name of
the symbol to define, and the textual value of the symbol. If the second
argument is empty, the symbol is defined, but has an empty value.

The earlier interpretation of a Yodl symbol as a logical flag can still be
used, but allowing it to obtain textual values greatly simplifies various Yodl
macros. 

    Example:
        verb(
    DEFINESYMBOL(Yodl)(Your own document language)
    DEFINESYMBOL(Options)()
        )

