a=[(1,2)]
b=[(3,4),(5,6)]
a.append(b)= [ (1,2) , [(3,4),(5,6)] ]
a.extend(b)= [ (1,2), (3,4), (5,6) ]
c='asdf'
a.append(c)=[(1, 2), 'asdf']
a.extend(c)=[(1, 2), 'a', 's', 'd', 'f']
