def check_tags(doc):
s = Stack()
tags = Stack()
i = 0
stub = ""
which = None
balanced = True
while balanced and i < len(doc):
if doc[i] == "<":
if s.is_empty():
s.push("<")
if doc[i+1] == "/":
which = "close"
else:
which = "open"
else:
balanced = False
if not s.is_empty():
if doc[i] == ">":
s.pop()
if which == "open"
tags.append(stub)
stub = ""
elif which == "close":
last = tags.pop()
if stub != last:
balanced = False
which = None
stub = ""
else:
stub += doc[i]
i += 1
if balanced and s.is_empty() and tags.is_empty():
return True
return False
