Problem:
 f(a()) -> f(f(a()))
 f(x) -> f(a())

Proof:
 Church Rosser Transformation Processor (no redundant rules):
  strict:
   f(x) -> f(a())
   f(a()) -> f(f(a()))
  weak:
   
  critical peaks: 2
   f(a()) <-0|[]- f(a()) -1|[]-> f(f(a()))
   f(f(a())) <-1|[]- f(a()) -0|[]-> f(a())
  Closedness Processor (*feeble*):
   
   Qed