YES

We show the termination of the TRS R:

  f(a(),f(x,a())) -> f(x,f(f(a(),a()),a()))

-- SCC decomposition.

Consider the dependency pair problem (P, R), where P consists of

p1: f#(a(),f(x,a())) -> f#(x,f(f(a(),a()),a()))
p2: f#(a(),f(x,a())) -> f#(f(a(),a()),a())
p3: f#(a(),f(x,a())) -> f#(a(),a())

and R consists of:

r1: f(a(),f(x,a())) -> f(x,f(f(a(),a()),a()))

The estimated dependency graph contains the following SCCs:

  {p1}


-- Reduction pair.

Consider the dependency pair problem (P, R), where P consists of

p1: f#(a(),f(x,a())) -> f#(x,f(f(a(),a()),a()))

and R consists of:

r1: f(a(),f(x,a())) -> f(x,f(f(a(),a()),a()))

The set of usable rules consists of

  (no rules)

Take the reduction pair:

  matrix interpretations:
  
    carrier: N^3
    order: lexicographic order
    interpretations:
      f#_A(x1,x2) = ((0,0,0),(1,0,0),(1,1,0)) x1 + ((1,0,0),(0,1,0),(0,1,1)) x2
      a_A() = (0,2,1)
      f_A(x1,x2) = ((0,0,0),(1,0,0),(0,1,0)) x1 + (0,1,1)

The next rules are strictly ordered:

  p1

We remove them from the problem.  Then no dependency pair remains.