tt(POPSYMBOL) is used to remove the topmost symbol from the symbol stack. 
The values of symbols may be pushed on a stack using
lref(PUSHSYMBOL)(PUSHSYMBOL). To remove the topmost element of a symbol's
stack tt(POPSYMBOL) is available. 

tt(POPSYMBOL) expects one argument: the name of the symbol to pop. The
previously pushed value then becomes the new value of the symbol. 

A symbol's value may be popped after defining it, after which its stack is
empty. In that case, using the symbol (although the symbol's name is still
defined) is considered an error.

    Example:
        verb(
    DEFINESYMBOL(YEAR)(This happened in 1950)
    POPSYMBOL(YEAR)
    COMMENT(YEAR now has an undefined value)
        )
