另一种就是使用序列中字符串的lower()方法，如果有耐心也可以使用translate()方法。把元素改为小写。
names = ['Adam', 'Lisa', 'Bart', 'Paul']
lis = [item.lower() for item in names] #将列表元素改为小写
s = set(lis)
print 'adam' in s
print 'bart' in s
for x in ['1','2','3','4','5','6','7','8','9']:
for y in ['1','2','3','4','5','6','7','8','9']:
if x>=y:
continue
print x+y
s = set(['Adam', 'Lisa', 'Paul'])
L = ['Adam', 'Lisa', 'Bart', 'Paul']
for i in L:
if i in s:
s.remove(i)
else:
s.add(i)
print s
