'Returns and prints time elapsed since last
tic(), tac() or toc() whichever occured last.'

def tac(self):

    import time
    if (self.start is None):
        raise Exception('tac()    without    tic()')
    t = time.time()
    elapsed = (t - self.last)
    self.last = t
    print 'TAC:    Elapsed:    {}    seconds.'.format(elapsed)
    return elapsed
