YES

   1: f(x1) -> x1
   2: f(x1) -> f(f(x1))

@Rule Labeling
--- R
   1: f(x1) -> x1
   2: f(x1) -> f(f(x1))

--- S
   1: f(x1) -> x1
   2: f(x1) -> f(f(x1))