Which of the following statement is not among the Peano's axioms?
1
If x is a number, the successor of x is a number.
2
Zero is a number
3
If P and Q are two numbers that are the same, and R and S are also the same, P + R is same as Q + S.
4
If a set S of numbers contains zero and also the successor of every number in S, then every number is in S.