We are left with following problem, upon which TcT provides the
certificate YES(?,O(n^1)).

Strict Trs:
  { f(x, x) -> a()
  , f(g(x), y) -> f(x, y) }
Obligation:
  innermost runtime complexity
Answer:
  YES(?,O(n^1))

The input was oriented with the instance of 'Small Polynomial Path
Order (PS)' as induced by the safe mapping

 safe(f) = {2}, safe(a) = {}, safe(g) = {1}

and precedence

 empty .

Following symbols are considered recursive:

 {f}

The recursion depth is 1.

For your convenience, here are the satisfied ordering constraints:

       f(x; x) > a()    
                        
  f(g(; x); y) > f(x; y)
                        

Hurray, we answered YES(?,O(n^1))