fib(0()) -> 0()
+(0(),x) -> x
fib(s(0())) -> s(0())
dfib(0(),x) -> x
+(s(x),y) -> s(+(x,y))
dfib(s(0()),x) -> s(x)
+(+(x,y),z) -> +(x,+(y,z))
fib(s(s(x))) -> +(fib(x),fib(s(x)))
dfib(s(s(x)),y) -> dfib(s(x),dfib(x,y))