tt(PUSHWSLEVEL) is used to start another lifetime of the white-space level
pushing the level's current value on a stack. See also section 
lref(INCWSLEVEL)(INCWSLEVEL)

tt(PUSHWSLEVEL) expects one argument, the new value of the white-space
level. This value may be specified as a numerical value or as the name of a
counter. The argument may be empty, in which case the new value will be 
zero. 

    Example:
        verb(
    COMMENT(Assume WS level is zero)
    
    PUSHWSLEVEL(1)
    COMMENT(WS level now equals 1)

    POPWSLEVEL()
    COMMENT(WS level now equals 0 again)
        )

