(VAR x y z)
(RULES
-(0,y) -> 0
-(x,0) -> x
-(x,s(y)) -> if(greater(x,s(y)),s(-(x,p(s(y)))),0)
p(0) -> 0
p(s(x)) -> x
)
(COMMENT Example 4.13 (Difference) in \cite{SK90})