tt(PUSHSYMBOL) is used to start another lifetime for a symbol, pushing its
current value on a stack. A stack is available for each individual symbol.

tt(PUSHSYMBOL) expects two arguments: the name of the symbol to push and its
new text after pushing. When the second argument is an empty argument, the new
text will be empty. The new text may be specified as a literal text, or as the
name of an existing symbol. Specify the name of the symbol twice to merely
push its value, without modifying its current value.

    Examples:
        verb(
    DEFINESYMBOL(YEAR)(This happened in 1950)
    PUSHSYMBOL(YEAR)(This happened in 1962)
    COMMENT(YEAR now has the value `This happened in 1962' and a 
            pushed value of `This happened in 1950')
        )
