(VAR x y u v)
(RULES
f(TRUE, x, y) -> fNat(x >= 0 && y >= 0, x, y)
fNat(TRUE, x, y) -> f(x > y, x, round(y+1))
round(x) -> if(x % 2 = 0, x, x+1)
if(TRUE, u, v) -> u
if(FALSE, u, v) -> v
)