3 A Chocolate Game Bar $CB(f,y,z)$ whose Grundy Number is $G(\{ y,z\} )=y \oplus z$

3.1 A Sufficient Condition for a Chocolate Bar $CB(f,y,z)$ to have the Grundy Number $G(\{ y,z\} )=y \oplus z$
3.2 A Necessary Condition for a Chocolate Bar to have the Grundy Number $y \oplus z$