YES Problem: a(b(x1)) -> b(b(a(x1))) c(b(x1)) -> b(c(c(x1))) Proof: String Reversal Processor: b(a(x1)) -> a(b(b(x1))) b(c(x1)) -> c(c(b(x1))) WPO Processor: algebra: Sum weight function: w0 = 0 w(a) = 2 w(c) = w(b) = 0 status function: st(c) = st(a) = st(b) = [0] precedence: b > c ~ a problem: Qed