(VAR x xs y z )
(RULES 
        plus(x, y) -> plusIter(x, y, 0)
        plusIter(x, y, z) -> ifPlus(le(x, z), x, y, z)
        ifPlus(true, x, y, z) -> y
        ifPlus(false, x, y, z) -> plusIter(x, s(y), s(z))
        le(s(x), 0) -> false
        le(0, y) -> true
        le(s(x), s(y)) -> le(x, y)
        sum(xs) -> sumIter(xs, 0)
        sumIter(xs, x) -> ifSum(isempty(xs), xs, x, plus(x, head(xs)))
        ifSum(true, xs, x, y) -> x
        ifSum(false, xs, x, y) -> sumIter(tail(xs), y)
        isempty(nil) -> true
        isempty(cons(x, xs)) -> false
        head(nil) -> error
        head(cons(x, xs)) -> x
        tail(nil) -> nil
        tail(cons(x, xs)) -> xs
        a -> b
        a -> c
        
)