YES (VAR x1 x0 x y z) (RULES i(f(x1,x0)) -> f(i(x0),i(x1)) a(b(a(x0))) -> b(a(b(x0))) i(i(x0)) -> x0 f(x1,f(i(x1),x0)) -> x0 f(i(x1),f(x1,x0)) -> x0 i(e()) -> e() b(b(x)) -> x a(a(x)) -> x f(e(),x) -> x f(x,e()) -> x f(i(x),x) -> e() f(x,i(x)) -> e() f(f(x,y),z) -> f(x,f(y,z)) ) (COMMENT Termination is shown by ELPO with interpretations on natural numbers f_A(x1,x2) = x1 + x2 + 1 i_A(x1) = x1 e_A = 1 a_A(x1) = x1 + 2 b_A(x1) = x1 + 1 i#_A(x1) = 0 e#_A = 0 a#_A(x1) = x1 and precedence: i > f > e > a > b )