Problem:
 b() -> f(f(b()))
 c() -> b()
 f(f(c())) -> a()

Proof:
 Uncurry Processor:
  b() -> f4(f(),f4(f(),b()))
  c() -> b()
  f4(f(),f4(f(),c())) -> a()
  Ground Confluence Processor:
   UN by decision procedure.