1. Question 6 in Chapter 2 (page 72)
2. When we talk about termination, we require a "decreasing sequence of natural numbers" to conclude that the sequence is finite. What happens if we remove
the second requirement and just require that we have a decreasing sequence of integer numbers?
3. A string s is called a palindrome if for every natural number i from 0 
to len(s)-1, s[i] == s[len(s)-1-i]. For exmple abba, 0123210 and 33 are all
polindroms. 
Prove that if the precondition for palCheck(s) is true, the postcondition 
will be satisfied. 

def palCheck(s) : 
# Precondition: s is a string 
# Postcondition: Returns True if s is a palindrome, False otherwise 
 if len(s) < 2 : return True 
 else : 
 s_inner = s[1:len(s)-1] # subarray s[1..len(s)-2] 
 return s[0] == s[len(s)-1] and palCheck(s_inner) 

4. Give an example of a language L over alphabet {0,1} that is infinite and
such that the compliment of L is also infinite.