Hint
As indicated by the inductive defintion, there are two appearances that numerals in the succnotation can have. They are either 0 or they are of the form succ(X) where X is again a numeral.
HintAs indicated by the inductive defintion, there are two appearances that numerals in the succnotation can have. They are either 0 or they are of the form succ(X) where X is again a numeral. 
