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+'''
+The nested brackets problem is a problem that determines if a sequence of
+brackets are properly nested. A sequence of brackets s is considered properly nested
+if any of the following conditions are true:
+
+- s is empty
+- s has the form (U) or [U] or {U} where U is a properly nested string
+- s has the form VW where V and W are properly nested strings
+
+For example, the string "()()[()]" is properly nested but "[(()]" is not.
+
+The function called is_balanced takes as input a string S which is a sequence of brackets and
+returns true if S is nested and false otherwise.
+
+'''
+
+
+def is_balanced(S):
+
+stack = []
+
+for i in range(len(S)):
+
+if S[i] == '(' or S[i] == '{' or S[i] == '[':
+stack.append(S[i])
+
+else:
+
+if len(stack) > 0:
+
+pair = stack.pop() + S[i]
+
+if pair != '[]' and pair != '()' and pair != '{}':
+return False
+
+else:
+return False
+
+if len(stack) == 0:
+return True
+
+return False
+
+
+def main():
+
+S = input("Enter sequence of brackets within quotes: ")
+
+if is_balanced(S):
+print S, "is balanced"
+
+else:
+print S, "is not balanced"
+
+
+if __name__ == "__main__":
+main()