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

Proof:
 Uncurry Processor:
  a() -> b()
  a() -> d()
  b() -> a()
  c() -> a()
  c() -> b()
  Ground Confluence Processor:
   confluent by decision procedure.