'Rollback the instruction stream for a program so that
the indicated instruction (via instructionIndex) is no
longer in the stream.  UNTESTED!'

def rollback(self, *args):

    if (len(args) == 2):
        programName = args[0]
        instructionIndex = args[1]
    elif (len(args) == 1):
        programName = self.DEFAULT_PROGRAM_NAME
        instructionIndex = args[0]
    else:
        raise TypeError('Invalid    arguments')
    p = self.programs.get(programName, None)
    if (p is not None):
        self.programs[programName] = p[self.MIN_TOKEN_INDEX:instructionIndex]
