YES We show the termination of the TRS R: sum(|0|()) -> |0|() sum(s(x)) -> +(sum(x),s(x)) sum1(|0|()) -> |0|() sum1(s(x)) -> s(+(sum1(x),+(x,x))) -- SCC decomposition. Consider the dependency pair problem (P, R), where P consists of p1: sum#(s(x)) -> sum#(x) p2: sum1#(s(x)) -> sum1#(x) and R consists of: r1: sum(|0|()) -> |0|() r2: sum(s(x)) -> +(sum(x),s(x)) r3: sum1(|0|()) -> |0|() r4: sum1(s(x)) -> s(+(sum1(x),+(x,x))) The estimated dependency graph contains the following SCCs: {p1} {p2} -- Reduction pair. Consider the dependency pair problem (P, R), where P consists of p1: sum#(s(x)) -> sum#(x) and R consists of: r1: sum(|0|()) -> |0|() r2: sum(s(x)) -> +(sum(x),s(x)) r3: sum1(|0|()) -> |0|() r4: sum1(s(x)) -> s(+(sum1(x),+(x,x))) The set of usable rules consists of (no rules) Take the monotone reduction pair: lexicographic combination of reduction pairs: 1. lexicographic path order with precedence: precedence: s > sum# argument filter: pi(sum#) = [1] pi(s) = [1] 2. lexicographic path order with precedence: precedence: s > sum# argument filter: pi(sum#) = [1] pi(s) = [1] 3. lexicographic path order with precedence: precedence: s > sum# argument filter: pi(sum#) = [1] pi(s) = [1] The next rules are strictly ordered: p1 r1, r2, r3, r4 We remove them from the problem. Then no dependency pair remains. -- Reduction pair. Consider the dependency pair problem (P, R), where P consists of p1: sum1#(s(x)) -> sum1#(x) and R consists of: r1: sum(|0|()) -> |0|() r2: sum(s(x)) -> +(sum(x),s(x)) r3: sum1(|0|()) -> |0|() r4: sum1(s(x)) -> s(+(sum1(x),+(x,x))) The set of usable rules consists of (no rules) Take the monotone reduction pair: lexicographic combination of reduction pairs: 1. lexicographic path order with precedence: precedence: s > sum1# argument filter: pi(sum1#) = [1] pi(s) = [1] 2. lexicographic path order with precedence: precedence: s > sum1# argument filter: pi(sum1#) = [1] pi(s) = [1] 3. lexicographic path order with precedence: precedence: s > sum1# argument filter: pi(sum1#) = [1] pi(s) = [1] The next rules are strictly ordered: p1 r1, r2, r3, r4 We remove them from the problem. Then no dependency pair remains.